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What I noticed is, that formatting a floppy disk takes three times as much time for the same data amount as writing a file to the disk normally. The accoustically perceivable jumps of the head between the eighty tracks are significantly slower on a full format than on a normal write of an 1.44 MB file which matches the MF-2HD.

If I expose a floppy disk to a magnet, the data gets corrupted, as expected.

  • I deleted the corrupted file.
  • I tried to write the same file again.

The floppy disks fail being written to. If the write succeeds, the new data after magnet exposure is also corrupted. But the floppy disk is not physically damaged.

  • After quick format: Data still corrupted after write.
  • After full format: New data can be written to the floppy disk just as usually.

I would appreciate a technical explaination. I am just curious.

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  • 9
    You would not expect a quick format to repair corruption from magnetic fields as it doesn't recreate the track and sector sync markers. That is why it is quick!
    – Chenmunka ♦
    Commented Jan 15, 2018 at 11:00
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    @Chenmunka Careful not to post answers as comments!
    – wizzwizz4 ♦
    Commented Jan 15, 2018 at 19:42
  • @WizzWizz4 I reposted his comment as answer because it was first only posted as comment.
    – neverMind9
    Commented Jan 15, 2018 at 22:55
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    @TechLord That's fine; I just got really confused about the situation.
    – wizzwizz4 ♦
    Commented Jan 16, 2018 at 7:42
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    Also in really cold weather floppy disks can stop working I remember having to wait for one of our pdp11's to warm up in a bitterly cold winter - I think we may have had to put a fan heater in the room.
    – Neuromancer
    Commented Jan 16, 2018 at 13:36

3 Answers 3

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A formatted disk contains markers which identify the start of each track and the start of each sector within the track. These markers are fixed magnetic sequences that are picked up by the drive electronics so that it knows where the sectors are.

On a full format, a completely blank disk has these markers written. This takes time as the drive must apply the markers, and check that they are correct.

On a quick format, the electronics assume that the markers are still there. The process is simply to mark the sectors as empty. Usually the sectors are overwritten with zeros, but not necessarily.

In both cases a blank directory is placed at the head of the disk by the operating system.

If you erase your disk, or part of it, using a magnet, the track sector markers are destroyed. A quick format won't find them and therefore the format and/or any write to the disk will fail.

There is some good information on track formatting of different floppy variants here. The low-level track & sector markers were often fiddled with by some software as part of a copy protection scheme.

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    Just nitpicking: "...picked up by the drive electronics...": no, by the computer's floppy disk controller, the drive just delivers a digital signal representing the magnetisation of the surface. "On a quick format, the electronics assume that the markers are still there.": Here it's the OS that assumes this fact, and only writes a new root directory structure into the header sectors, but leaves all sector markers and data sector contents unchanged.
    – Ralf Kleberhoff
    Commented Jan 15, 2018 at 12:19
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    @RalfKleberhoff: Good point.
    – Chenmunka ♦
    Commented Jan 15, 2018 at 12:36
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    You say (of a "quick" format): "Usually the sectors are overwritten with zeros, but not necessarily." My experience (dominated by, but not exclusively, IBM-PC) was that usually it's the other way around: most data sectors aren't touched; only those that hold the OS's notion of what's on the disk is wiped (FAT, root-directory etc.).
    – TripeHound
    Commented Jan 15, 2018 at 13:34
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    @TripeHound That matches my experience as well, and it wasn't always zeroes, e.g. CP/M used to fill newly-formatted sectors with 0xE5 bytes.
    – Ralf Kleberhoff
    Commented Jan 15, 2018 at 15:14
  • "If you erase your disk, or part of it, using a magnet, the track sector markers are destroyed" - As long as we're nitpicking, I feel it would be more correct to say "may be destroyed". You could get lucky with your magnet and miss the track/sector markers. After all, "using a magnet" is the normal and routine way of erasing data from a floppy disk. It's just normally a very small and very precisely controlled magnet that does the job.
    – aroth
    Commented Jan 16, 2018 at 9:34
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The main thing is that there is no such thing as a "quick format" - That term is entirely misleading terminology invented by Microsoft. Quick Format doesn't "format" anything.

What MS calls a "Quick format" is rather a "wipe directory" - It marks all sectors as unused and rewrites only the FAT and root directory. That process visits only a very limited number of tracks on the disk - that is why it is so fast.

A proper format of a disk needs to re-write all sector markers on the disk, so all tracks and sectors have to be touched by the write heads.

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The magnetic field doesn't destroy only the file contents, but the contents of the directory blocks and the block allocation table, as well as the sector markers on the disk itself. A full sector on a disk doesn't just contain the payload, but - depending on the system - a 'sync' marker denoting its start, the sector number itself and a checksum.

So deleting and rewriting the file most probably does not work - the only correct way is to backup the undamaged files, do a full format of the disk, and then rewrite the files.

A bit offtopic, but worth mentioning: Modern hard drives contain essential operational data on reserved sectors, if that becomes unreadable for any reason, the hard drive is essentially bricked - all you'd hear is that 'click of death'.

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  • Why don't hard drives provide any way of restoring that operational data without special equipment, say, through a "low level format" command that can be sent from a PC? Would it actually add a non-negligible amount of technical complexity, or is it only because they'll sell more hard drives if people replace rather than repair them?
    – Sparkette
    Commented Mar 14, 2021 at 4:44
  • @Sparkette: Modern hard drives have a mechanism that sets mechanical position by varying the amount of current in a coil. It would be impractical to establish a fixed relationship been coil current and head position that wouldn't vary unacceptably with temperature. Instead, disks have factory-written markers that report the head position, and adjust the coil current until the head is in the right place. Writing the markers requires equipment which is much more finely calibrated than anything in a consumer-grade hard drive.
    – supercat
    Commented Apr 12 at 21:21

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