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Working with Disks

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An intro to floppy disks and floppy drives

A few years ago I spent some time archiving my old floppy disks and I was reminded how much I hate them and how frustrating they can be. Cheap and large hard drives have really made computing so much more productive and enjoyable, and SSDs and thumbdrives have eliminated the fragile mechanical parts of data storage. However, older systems still use floppy drives and understanding how they work makes working with them much easier. Here is some background information on floppy drives and disks with some tricks for using them that you might find useful. While this page is geared to IBM PCs and compatibles, the basic concepts apply to other vintage computers.

The first floppy disks and drives were invented by Alan Shugart at IBM in 1971. Those original floppy disks were 8 inches in diameter and were designed to allow IBM to distribute microcode updates cheaply. The original versions stored 250KB which was a fairly large amount at the time.

Alan Shuggart went on to found Shugart Associates, which in 1976 introduced a smaller version of the floppy disk that was 5.25 inches in diameter. The new market for floppy drives led other companies to jump in, each with their own ideas on how they should work, leading to a wide variety of incompatible disks and drives. (The variations included things like how recording was done, how sectors were marked, rotational speed, track pitch, etc.)

When IBM created the IBM PC they chose to use a single sided, soft sectored, double density drive capable of storing 160KB on each side of a disk. Shortly after IBM started to ship double sided drives which combined with a software change allowed for 360KB of storage on a disk. The IBM PC, PC XT, and PCjr used that standard. Later machines introduced floppy drives with higher capacities or different sizes - the IBM PC AT used 1.2MB 5.25" drives while the IBM PC Convertible used 720KB 3.5" drives). The standard 1.44MB 3.5" that everybody recogizes as the "save" icon came later, and other variations such as the 2.88MB disk and various "super drives" were introduced but did not really catch-on.

Other personal computers such as the Apple ][, the Commodore 64, the Macintosh, etc. all of course did their own thing too. It was a mess, which is what makes it so interesting.

A floppy disk stores data on a flexible round sheet that has a surface coated with a compound that can store magnetic flux transitions. The media is supported and protected by an outer jacket. The recording technology is very similar to magnetic tape, except that the diskette is optimized for random access across the surface instead of streaming speed.

Disks are either single sided or double sided. Most disks are double sided, meaning that both surfaces are suitable for recording. Early disks may be single sided, meaning that only one surface was certified for data storage.

A floppy drive is a storage device that contains a spindle motor, read and write heads that move linearly on a track, a motor to move the heads, sensors, and support circuitry. When a floppy disk is inserted and mounted on the spindle the floppy drive can rotate the media in the protective jacket and the heads can access most parts of the media through an access slot in the jacket. Sensors are used to determine if the floppy disk is single or double sided, where track 0 is, is the floppy disk write protected, etc. The heads are in direct contact with the media, so eventually the media will wear out. This also makes the heads prone to picking up contaminants, like shedding oxide from the media.

Data is laid out on the surface of a disk in tracks and sectors. A track is a concentric ring at a fixed location from the center of the disk. A sector is a small area of a track. Unlike a vinyl record (which was current technology when disks were invented) or a CD-ROM, there are multiple tracks on a disk, not just one spiral. A stepper motor in the disk drive positions the drive heads to the desired track.

Data is written to the disk a sector at a time. There is no way to change just one byte of data - to change one byte, the sector containing the byte must be read, the new byte needs to be put in the buffer holding the sector, and the entire sector must be written out. Sector size is set by the drive and controller. IBM PC clones generally use a 512 byte sector.

A drive "formats" a disk before it stores data on it. The formatting process can be thought of as laying out "street signs" so that data stored on the disk can be found later. The "street signs" are numbers, identifying areas on the disk. Disks can be formatted in many different ways, but in general only a few well-known formats are used. The format used depends on the drive and the disk.

IBM introduced the PC with a single sided, double density 5.25" drive. This drive formatted disks to a 160KB capacity using just one side of the disk. If you wanted to use the other side of the disk you had to remove it and flip it over.

The original single sided drives were quickly replaced with double sided, double density drives capable of formatting a disk to 360KB in size. These drives also used 40 tracks per side, but the sectors per track increased to 9 and both sides of the disk could be used without flipping the disk over. By the time the IBM PC XT and PCjr were introduced double sided, double density drives were standard.

Below are the parameters for standard formats used by these drives:

The sector size for IBM compatible computers is 512 bytes. Computing the capacity of a disk is just a matter of multiplication:

Sectors per track * Tracks * Sides * 512 = Total Bytes

For example, the 360 KB format shown above uses 9 sectors per track * 2 sides * 40 tracks* 512 byes per sector, for a total of 368640 bytes on a disk. (368640 = 360 KB)

A 5.25" disk is fragile compared to a newer 3.5" disk. The outer jacket is very flexible, and there is a large oval shaped hole where the media is completely exposed. The 5.25" disk is write protected by placing an opaque "tab" over the write enable notch on the side of the disk. (Scotch tape might not work because both optical and mechanical sensors were used on the write protect mechanism!).

Be careful when inserting a 5.25" disk! When you close the drive door or lever, the drive clamps onto the disk at the center. Close the door or lever slowly to allow the disk to center on the spindle; if you close it too fast you might not get the disk clamped on-center, which will make it impossible for the drive heads to follow the circular tracks!

The design flaws of the 5.25" disk were corrected on the 3.5" disk. The 3.5" disk has a harder casing, the media is protected by a shutter, and the hub ring was replaced by a much sturdier mechanism.

These disks generally spin at 300 RPM, or five times per second. The data rate from the controller is 250kHz, which means each bit takes 4 microsends to read or write. You only have 0.2 seconds to read or write a track, so at this data rate you can only store 50,000 bits per track. At 40 tracks per side that works out to 2,000,000 bits per side, or 250KB per side/500KB per disk. There is some overhead for marking the sectors on the disk and leaving a gap between sectors, so the effective capacity is reduced to 360KB per disk.

In 1984 IBM introduced its first high density drive. This drive was designed to use "high density" media, which allowed for data to be packed more tightly on the disk. This in turn increased storage capacity. Below are the parameters for the new format that this drive used for high density media:

High density media requires a different amount of magnetic force to write than double density media requires. Although all disk media looks alike, the media is different. High density drives can use double density media because they detect the media and change how they write to it, preserving backwards compatibility.

To support the additional data density the drive controller has to send data at a faster rate. High density drive controllers send data at 500kHz, which is twice the speed of double density drive controllers. With the faster data rate there is just 2 microseconds to read or write a bit of data.

These drives spin high density media at 360 RPM, which gives you a full track access time of 0.1666 seconds. That works out to 83,333 bits per track. These drives have 80 tracks so the unformatted capacity is 1.666MB, but just 1.2MB after overhead.

You may have noticed that the number of tracks doubled but the density of each track did not. That is due to the faster spin rate that is used for high density media. Spinning the media faster reduces the average wait time when trying to read data, at the expense of data density. If these drives had doubled the data rate and kept the same speed they would have held 4x as much as a double density 5.25" drive. (To be fair, the faster spin rate might have been chosen to match the capabilities of the media. Spining at 300 RPM might have exceeded the ability of the media to store the magnetic flux transitions.)

To double the number of tracks the heads have to be at least half the width as the heads of a 40 track drive. Instead of packing 48 tracks per inch, these drives have to fit 96 tracks per inch. This can lead to compatibility problems when writing double density disks that use the older, wider heads. (More on that problem later.)

The standard 5.25" disk was fragile and inconvenient to work with. The 3.5" drive was introduced as an alternative to the 5.25" drive. It was especially popular for portable systems, where space was at a premium. Below are the parameters for the double density 3.5" drive:

This format looks amazingly similar to the format used by the double density 5.25" drive, except that it has twice the number of tracks. This made it easy for computer manufacturers to transition to this format - it was a very minor change. They spin at the same speed and use the same data rate from the controller. Electrically the new drives looked like standard 5.25" double density drives.

The double density 3.5" disk was successful; the disks held more than the original double density 5.25" disks, the media was better protected, and they were convenient to use. Moving to high density media made for the most successful type of disk in personal computer history - the ubiquitous 1.44 MB disk. Below are the parameters for the high density 3.5" drive:

Like the high density 5.25" drive, these drives need high density media to write the higher capacity format. Unlike the high density 5.25" drives, these drives only spin at a single speed (300 RPM). Using the faster data rate the drives take 2 microseconds per bit, laying down 100,000 bits per track. That gives them an unformatted capacity of 2MB, or a formatted capacity of 1.44MB. With double the data rate and the same rotational speed they hold twice as much as their predecessor 3.5" double density drives, and four times as much as the 5.25" double density drives.

Density is a description of the physical media and how much magnetic force is required to write data to the media. A double density disk has a different coating and has a lower maximum storage capacity than a high density disk. A quad density disk has yet a different coating that enables even more data to be stored on the media.

When speaking about drives, density refers to the types of media a drive can write to. A double density drive can only be used with double density media, while a high density drive can be used with both high density and double density media. To take advantage of the media, the drive has to be able to accept data from the controller at a faster rate. Double density drives generally use a data rate of 250kHz while high density drives use a data rate of 500kHz. The higher density drives maintain backwards compatible with the lower density media by varying the data rate, magnetic force used to write, and possibly the rotation speed.

Format describes how data is logically written onto the media. Formatting a disk marks where sectors are going to be on the disk. A high density disk is capable of storing more data in the same physical space, so it can have more sectors per track than a double density disk. While most people will choose a format that maximizes the capacity of the disk, that is not a requirement - you can choose to format a diskette with a "relaxed" format that does not push the limits of the media. Here are some concrete examples:

• A 3.5" double density disk is usually formatted to 720KB using 80 tracks, 9 sectors per track, and two sides. It is perfectly legal to format it to 360KB by using only 40 tracks, thus leaving half of the disk unused.
• A 3.5" high density disk is usually formatted to 1.44MB using 80 tracks, 9 sectors per track, and two sides. It is perfectly legal to format it to 720KB by using all 80 tracks, but cutting the sectors per track in half to 9.
• A 5.25" double density disk is normally formatted to 360KB using 40 tracks, 9 sectors per track, and two sides. But there were older IBM PCs out there that only had single sided drives. If they were running DOS 1 or 1.1 they might only try to use 8 sectors track. So for compatibility reasons many 5.25" disks were only formatted to 160KB (40 tracks, 8 sectors per track, 1 side) to ensure they would be readable on those very first PCs.

Note: I pretty much ignore "quad density" or "extended density" media in this discussion because they are unusual to find. There were some 2.88MB floppy drives out there, but CD-ROMs basically killed those off. The general rules are the same though; it's a different media type from the others.

Most people with a 3.5" floppy drive are used to having that floppy drive work in any other floppy drive. While that is true for most PCs built in the 1990s and 2000s, it was not true for older computers. Some care must be taken when working with older systems or you can corrupt your data.

Here are the factors to consider:

• Disk density: is the disk double or high density?
• Drive density: does the drive support double or high density disks?
• Track width: How wide are the heads that are going to be usd to read or write the disk?

Let's start with the easy case first: density differences.

The recording surface of a double density disk is different than the recording surface of a high density disk, and they are not directly interchangable. Floppy drives mask over this difference by detecting and adjusting their write current depending on the media that is being used.

In short, don't try to use a high density disk in a double density drive or try to force a double density disk to act as a high density disk.

How can you tell double density media from high density media? The disk should be labeled with either a "DD" or an "HD". Sometimes the words "Double Density" or "High Density" are used. The formatted capacity may also be specified. In the absense of any markings, a double density 5.25" disk usually has a hub ring which reinforces the disk where the disk drive clamps the media while a high density disk does not usually have a hub ring. You can't see a hub ring on a 3.5" disk because the mechanism is different, so you just have to hope the disk is labelled correctly. On a 3.5" disk you can also tell a double density disk by the lack of a square hole on the opposite side of the write-protect mechanism. (High density disks have another hole, where double density disks do not.) (Thanks to Octavio Alvarez for the tip.)

Old double density drives always assume that they have double density media. They will try to write to a high density disk, but it will either fail or not work reliably. High density drives usually have a "media sensor" that will tell it what type of disk is inserted. This allows the drive to fail if it is told to format a low density disk with a high density format. Some disk drives and controller combinations did not implement this correctly so do not rely on the hardware to prevent you from making a mistake.

The important thing to remember is that density refers to the storage capacity of the media or the capability of the drive to use different media, while format is a logical construct that can vary widely. No matter what media is used, the drive still has to support it and use the correct levels of magnetic force to write data to the media.

Now for track width ...

Track width differences only apply to 5.25" drives. Remember, the first 5.25" drives started at 40 tracks and then high density drives used 80 tracks. To make that happen the track width had to be halved. 3.5" drives are always 80 track drives so their track width never changed.

Track width is a problem when writing a diskettes and moving them between double density and high density drives. Assume you are starting with a factor fresh, unformatted diskette with absolutely no data on it:

So the short story is don't write a double density diskette in a high density drive and then expect a double density drive to be able to read it. The narrower write head overwriting the wider preexisting data can confuse the wider drive head in the double density drive. You can reformat the disk in a double density drive and use it again, but you won't be able to write to it reliably from a high density drive because the tracks were written in a drive that has wider heads than the high density drive.

If you make this mistake it is not the end of the world.

• It might work, but it depends on the individual drives and how tolerant they are.
• A magnetic bulk eraser can restore the magnetic media to a state that resembles a never written or formatted disk.

If you must write double density media in a high density drive and read it in a double density drive you have to start with a factory fresh diskette that has never been formatted or used before. (Or buld erase it before formatting it in the high density drive.)

Every person who relied on floppy disks knows the value of a good backup. Floppies are very unreliable compared to hard disk technology or tape; they were designed for moving data between systems easily and cheaply, not for long-term archival storage.

There are many ways to make backup copies of your floppy disks. Here are the methods I use:

This is the most simple way to preserve data. Copying the files to a different device or media type is easy to do but it does have some potential pitfalls:

• File copying can miss hidden files which can be a problem for software that used hidden files.
• File copying misses system files, which are required for diskettes that you boot the machine from. Example: Copying a DOS boot disk by only using the copy command will not result in a bootable DOS disk.
• File copying doesn't work on copy protected diskettes, as that type of software generally depends on the physical characteristcs of the disk and file copies don't preserve those.
• File copying does not preserve deleted files, which may still be recoverable on the disk.
• File copying does nothing to detect corruption in the file at a later point.

Archivers are like file copying except that you can copy groups of files and the group is compressed to save space. The utility that archives the files also has a way to extract files from the archive. The archive file takes up less storage, is easier to manage than multiple files, and takes less time to transmit.

PkZip is the most common file archiver for old DOS systems. Other older file archivers include InfoZip, Tar, LhArc, Arc, Zoo, etc. Most of these formats are still supported by modern archivers today.

File archiving has most of the same problems as file copying. One key difference is that file archivers can detect corruption in the archive, so that you will at least know if the data has been corrupted since it was archived. Knowing the data was corrupted won't help you recover it, but if you have two copies of the archive then the other one is hopefully readable.

Diskcopy is a DOS command that makes an exact copy of your entire disk on another disk. All parts of the disk including hidden files, system files, and previously deleted files and slack areas are copied, making it suitable for analysis or forensics later on. And of course a bootable disk copied with Diskcopy will also be bootable, making it suitable for cloning your boot disks.

Diskcopy does not work with copy protected disks. Diskcopy also requires that the target and source disks are the same type and capacity. Diskcopy can not handle errors on the disks.

DOS Diskcopy is limited to reading and writing physical disks. A disk imager adds the ability to create a file that represents an image of a full disk, or allows you to recreate a disk from an image. Think of it like DOS Diskcopy, but now you can choose a file as either the source or the target of the operation.

Disk imagers are great for archiving diskettes because like DOS Diskcopy, they capture the entire disk including the boot sector, hidden files, and slack areas. But the image is a file that you can move around and copy as needed. Virtual machines, Linux and DOS emulators also can work directly with disk images, allowing you to minimize the use of physical disks.

These programs are like diskcopy, except that they can reproduce copy-protected disks. These programs were usually purchased to make backups of copy protected games. They can be used on non-copy protected disks that have read errors. (DOS Diskcopy can not handle this case.) Some of these programs only copy a disk onto another disk, while others can create disk image files that can be copied and moved around like large files.

While these were a good solution for handling copy-protected disks many years ago, most of them do not work on modern hardware. Modern hardware generally does not have a floppy drive controller in the chipset, and USB floppy devices are just generic storage devices to modern machines. Even older hardware that has a floppy controller in the chipset often does not work because of speed and timing differences between them and the original machines these programs were designed for.

Back when I was using a PCjr I used all of these methods except for disk images. (The PCjr only supports double density 5.25" drives and there was no other storage, so an image file could not be handled.) When it came time to preserve my disk collection I decided to use both Zip files and disk images. For each disk I create a disk image and a Zip file. The disk image is good for archival purposes, and the Zip file lets me retreive individual files quickly. Zip files are compressed as part of the archiving process, while disk images can be compressed separately to save space. (The easy thing to do is to make a Zip file of the disk image, which also adds the ability to detect data corruption in the disk image file.)

• DskImage is my own program for creating and writing disk images. I wrote it to handle disks that were developing read errors; aggressively retrying read errors often works to get a good read of a marginal disk sector. DskImage works on any DOS machine running DOS 2.1 or later and it lets you specify the tracks, sectors and sides allowing you to manipulate strangely formatted disks.
• RAREAD and RAWRITE are older utilities that can read and write disk images. There are many different versions floating around but they all basically do the same thing. (RAWRITE was often shipped with early Linux distributions so that you could create the required install disks.) A version of RAREAD and RAWRITE can be found here.
• The Linux dd command can be used to create or write an image file. Image files can be mounted on a directory using the loopback option of the mount command, allowing you to peek inside of the image and read files. (Some older DOS disks will be confusing to Linux and it will not handle them correctly, so this is not a perfect solution.)
• If you are an OS/2 user you may run into IBM's LOADDSKF and SAVEDSKF programs. These create and write disk images but they use their own format that includes some metadata and has compression, so they are not compatible with other disk imaging programs. LOADDSKF and SAVEDSKF can be found here.
• WinImage is a powerful program that can work with many different types of media and image files.

For copy protected disks you originally had very few options:

• The Central Point Option Board: Years ago this was the ultimate disk imager. The Central Point Option Board is an ISA card that was installed between the machine's floppy controller and the floppy drive, allowing it to intercept signals and data and control the floppy drive. As a result, it required period hardware such as an IBM PC, XT or AT, and highly compatible machines. Option Boards are rare and hard to find now.
• Teledisk is a program that can read and write image files of copy-protected diskettes. TeleDisk was shareware years ago and old copies are still available for download.

Things have improved a lot in the past few years, and now there are modern projects that are capable of imaging floppy disks for archival purposes:

• The Greaseweazle is a USB attached device that can read the raw magnetic flux transitions from a drive, making it suitable for archiving basically any floppy disk that you can find - it is not just for IBM compatibles! The design is fully open.
• There are a few Raspberry Pi projects that allow you to interface a Raspberry Pi to a floppy drive so that you can read or write to the drives.

Whatever method you choose, remember to test it thoroughly! As with any backup, it is not good unless you have tested it.

Disks can (and will) go bad. Usually they start by developing "soft" errors, which are recoverable by retrying the operation several times. Eventually the soft errors turn to "hard" errors that can not be recovered.

How do you know when a disk is starting to go bad? Your first clue will be an error message like "Disk error reading Drive x, Abort, Retry, or Ignore?" When you start to see that happen to a disk, it is time to do something.

• Try the disk in another drive to ensure that the disk is the problem, not the drive. (The drive heads may be dirty and need cleaning.) If the disk is fine in another drive then you should look at the next section on drive malfunctions.
• Try to copy the individual files that you need onto another disk or hard drive. If you encounter an error Answer "Retry" to the error message up to five times ... there is a chance that one of the retry attempts may succeed. (After three retries it is possible that a read will succeed, but it becomes less likely.)
• Try to make a copy of the disk onto another disk using diskcopy. (If the errors are severe enough, you might not make it through diskcopy.)
• Try to use a disk imager or a disk copying program. You may lose some data if the error is in one of your files and you can not get past it, but you will save the rest of the disk before it becomes a problem.

If you get your files off the disk safely, there are some steps you can take with the disk. It is not necessarily bad, it just may need some help:

• Did you write a double density 5.25" disk in a high density drive and then try to read it on a double density drive? If so, you have been bitten by the track width problem described earlier. Copy the data to a safe place and then reformat the disk in a double density drive to make it usable again.
• Are some sectors just getting weak? If so use a program like "Refresh" to detect and rewrite failing sectors. Writing a new strong signal may clear up some read errors.
• Copy the disk, reformat the disk, and then copy the data back onto the disk. This is similar to the suggestion above, except that you are processing every sector on the disk. You may find that format reports bad sectors - if it does the disk is possibly damaged. Bulk erasing it might help, but if there is wear on the disk surface then it should be retired.
• Assuming you have a good copy of the files, recopy the files back onto the disk. This is similar to the suggestion above, but it is not as complete. It is also a little risky as the new copy of the file might land on a different place on the disk, leaving a potential time bomb unaddressed.

It does not pay to try to reuse a floppy that format reports has bad sectors. If format can't handle the disk it has reached its end of life. The disk may be physically damaged, and you may lose more data in the near future.

Not every floppy disk error is caused by the floppy disk. The drives are all old now and they wear out too. Here some of the problems I have run into:

• Poor head positioning: The drive might not be positioning the drive heads correctly, making a disk look flakey. Sometimes there is gunk on the drive rails that can be cleaned away to improve this. Clean and lubricate the drive rails if you suspect this is happening. I use isopropyl alcohol for cleaning the rails and a silicone based lubricant to lubricate them. Both are applied using a Q-Tip. DO NOT SPRAY A CHEMICAL SUBSTANCE NEAR A DISKETTE DRIVE! You do not want foreign chemicals or materials on the drive heads, and spaying chemicals is a good way to damage things.
• The drive heads may be dirty. I clean the drive heads very gently using a Q-Tip soaked in isopropyl alcohol. A cleaning disk is good if you can find one, although a 5.25" cleaning disk is going to be hard to find now. Be very careful doing this; it is very easy to damage the drive heads when you are touching them with the Q-Tip.
• Disk clamping problems: If the drive does not clamp down on the disk well or if it clamps the disk off-center the disk will be unreadable because it will not spin in a circular pattern.
• Worn belts: The drive has to spin at a constant rotational speed within a narrow margin of error. If a belt is slipping or the drive motor is spinning at the wrong speed the spindle will spin at the wrong speed, making the disk unreadable.
• Sticking motors: A stepper motor is used to move the heads to the correct track, while a spindle motor is used to spin the disk at a constant speed of either 300 or 360 RPM. If motors are not exercised they can seize. If you think you have a seized motor, use some penetrating oil on it and let that sit for a few days before trying to exercise the motor by hand.

Be careful when attempting to clean or adjust a floppy drive. When in doubt, seek help from somebody who has more experience.

Created in July 8th 2001, Last updated January 17th, 2022
(C)opyright Michael B. Brutman, mbbrutman at gmail.com

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