I have a dozen or so 3.5" floppy disks from the 90's and most of them show bad blocks when I scan them with badblocks. If I do a low-level format, where the sectors are laid down again, will that make the floppies more reliable? I don't care about preserving the contents; just wondering if I should throw them out or keep trying to use them.

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    For all practical cases I know, it's not the floppies but the floppy drive which is broken.
    – Janka
    Dec 27, 2017 at 6:30
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    I've tried three floppy disk drives and they seem to agree, though.
    – Sydius
    Dec 27, 2017 at 6:48
  • There are simply too many reasons why a floppy disk might fail to be able to give a definitive answer to this. In case it helps - fine, if not, it doesn't help ;)
    – tofro
    Dec 27, 2017 at 10:18
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    On a related note, about 5 1/4" floppies. Avoid using regular DD (40 track, <=360KB) disks in a HD drive (80 track, 1.2MB). But if you have to, at least never format the disk in the HD drive; do it on a DD drive. The HD head is narrower than the DD head and formatting under the HD drive will leave behind the edges of the previous formatting bookkeeping. A DD drive likely won't be able to read the disk because it will see a mixture of both signals, and even on the HD drive it might be more sensitive to slight misalignments. It's best to just refrain from writing to a DD disk with a HD drive. Dec 27, 2017 at 16:36
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    A bit off topic, but anecdotally - I've been imaging my 3.5 floppies from my Amiga and early PC days (DOS3.3 to 6.22) and I've been amazed how well they have held up. Some of these disks are almost 30 years old with very little lost data - amazing!
    – Geo...
    Dec 28, 2017 at 0:55

4 Answers 4


It won't improve "disk longevity", but it will make the disks usable again. How long they'll stay usable depends on a lot of things.

The magnetic flux pattern on the floppy disk that is used to store the sector address, the begin and end markers, and the data itself, gets weaker over time. If you low-level format them, you'll also refresh the flux pattern that's used for "administrative" purposes (finding sector start and end), not only the data itself. So that's a good thing, and helps.

On the other hand, floppies may go bad for other reasons: The magnetic coating may come off gradually due to friction, and a detoriating sleeve material will speed up this process. If stored improperly, moisture can form between the sleeve and the floppy, which can lead to mold and other things that may detoriate the surface. And so on. Low level formatting won't help with any of this.

And then there's the whole issue of possible defects in the drive: Misalignment, dirt on the read-write-head which will make the read signal weaker, and also cause read errors, etc.

  • Also, the slightly different sector layout after a low level format could expose locations with media errors which were unused and ignored with the previous format.... Dec 27, 2017 at 17:17
  • @rackandboneman or it could obscure such locations by slightly shifting the sectors to some other place - nothing to gain from here. Either a disk works after a format, or it doesn't. You'll never know how reliably it does it before you do the format.
    – tofro
    Dec 27, 2017 at 18:06
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    I'll add my comment to this question, as it is the one accepted. What all answers to this question seem to miss to mention (or take as granted) is: Formatting a floppy writes some very important and fundamental informantion like sector markers, numbers, sync bits and boundaries to the disk that is never again rewritten until the next format. Writing a disk, even clearing it completely, does not touch that information. Re-formatting a disk does, however, refresh that information.
    – tofro
    Sep 22, 2019 at 9:40
  • @tofro: I thought I made that clear with "gets weaker over time" and "low-level format refreshes it", but maybe not clear enogh...
    – dirkt
    Sep 22, 2019 at 14:23

First of all, there is no such thing as not 'low-level' formating of a floppy. There's just formating. The misconception of a 'high-level' format (or what ever the counterpart would be) of a floppy is easy to receive from the MS Windows concept of 'fast format' - which simply clears FAT and root directory but doesn't really format anything - in conjunction with drive specific 'low level' formating introduced as buzzword with hard disks (*1).

Having said that, it was always a good idea to format a floppy before (re-) using it. In 'ye good old days' it was necessary to get best results, as every drive was aligned a bit different. Buying preformated disks was shunned on.

Floppies naturally loose their content (and format is also just content) over time. Also they are more likely to be stored in a less than perfect environment that can reduce their storage - mostly in fact by temperature changes than due magnatic fields.

So yes, don't be lazy, do it like grandpa, format any floppy before use.

*1 - Last sentence is taken (and a bit edited) from a great comment by @tofro

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    The misconception of a "high-level format" of a floppy is easy to receive from the MS Windows concept of "fast format" that simply clears the FAT and doesn't really format anything.
    – tofro
    Dec 27, 2017 at 12:52
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    You are rewriting sector indices etc (which you usually cannot do easily with a post-1980s HDD for good reasons), versus just creating a new filesystem (usually called a high level format). How low does it have to be for you? Dec 27, 2017 at 17:26
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    Quick format didn’t originate with Windows, DOS 6 FORMAT supported it, perhaps even DOS 5, and other tools supported it before that. Dec 27, 2017 at 20:18
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    (But that’s just a minor nitpick.) Dec 27, 2017 at 20:19
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    You’re right, Windows 3.1’s File Manager had it before DOS 6 was released. But DOS 5 had it too, along with unformat support ;-). Dec 27, 2017 at 22:55

Not really an answer but too long for a comment.

If a floppy drive is out of alignment when writing to the floppy, any other aligned drive will fail to read it. It may or may not fail to read any other floppy.

The disk heads also need cleaning once in a while. If they haven't been used for a while and they are internal drives, they will need vacuuming as well. The fans suck air through the system and that includes floppy drives so all the dust gets trapped in the floppy.

You can try formatting but if that doesn't work, just bin them.

Nowadays floppies are high security devices because none of the machines sold have a floppy drive and very few people own portable floppy drives.

  • Is it possible to realign and old 3.5 FDD drive in order to restore its proper functioning? Does it requires special tools or special skills? Sep 21, 2019 at 20:21
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    The special tool is an oscilloscope. I don't know what skills are required. I saw it being done about 35 years ago. The engineer probed a certain point and the scope showed a figure-of-8 but it was skew to one side. He then turned a screw somewhere until the figure-of-8 balanced out. That was on an 8 inch floppy drive. The 3.5 may be similar but being a softie, I have no experience of this.
    – cup
    Sep 22, 2019 at 6:17
  • @cpu Realignment also requires (IIRC) a specialy formatted alignment disk that has the correctly aligned tracks written on it so that you can match the head to these.
    – cjs
    Sep 23, 2019 at 5:08

I've never heard of a "low level" format for a floppy. The standard format, the one that goes ticka ticka ticka for about 30 seconds -- that is doing what hard drives call a low level format - blanking the media and rewriting the sector boundaries. It's not a particular challenge; sector bondaries are just more data.

There was eventually a "quick format" which simply zapped the FAT directory table and boot sector. But that was always a specified option, e.g. format a: /q

Some companies sold floppies pre-formatted, so you didn't need to spend 30 seconds. I wasn't much into those, being a 3-platform shop. But I can imagine if someone only ever bought those and did format /q by rote, they might think a normal/full/low-level format was the odd one.

Yes, a normal/full format may help with bad blocks, if the problem is the magnetic fields fading from age (as opposed to the media deteriorating, say).

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    I mean a format that writes the sector boundaries onto the disk, not just the file system.
    – Sydius
    Dec 28, 2017 at 19:37
  • @sydius Yes, I know you meant that. Floppy normal/full format assumes a truly blank, fully degaussed media (or one which was last formatted on Mac or Amiga, whose sector boundaries are unrecognizable to PC, and its sector boundaries must be rewritten/reestablished). Sep 22, 2019 at 16:39
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    @Harper No, the standard floppy format routines (which in modern terminology do a "low-level" format) rewrite each track entirely; they do not care what, if anything, was on the track before. I found that this FDC data sheet makes it clear what's really going on each disk track.
    – cjs
    Mar 1, 2020 at 15:53

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