recovery of a floppy disk
December 24, 2002

want to recover a dead hard disk instead? Try this..

This technique is for recovering data from a floppy disk with a dead FAT (although it could be modified for repairing the MBR as well).

The technique won't recover data from a dead sector. It will, however, recover a disk which, apart from the FAT, is fine.

What is a dead FAT? The File Allocation Table (FAT) is where DOS/Windows stores its index of what file is in what location on the disk. If the FAT becomes corrupted, although the files are fine, the disk may appear unusable - for example, you see a "General failure reading drive A:" message when you try and get a directory of the disk.

The wizards at Seattle Computer Products (or was that Digital Research?) had the foresight to build a redundant copy of the FAT into each disk. So if the first FAT becomes corrupted, the second can be used. Except, at least on the disk I just fixed, the second wasn't "kicking in" - perhaps that's only a feature of a Real operating system.

So, this technique is essentially about how to get the second FAT to kick in. For starters, I used a copy of Norton Utilities 9.0 for DOS for this. Windows versions may work - I didn't try them. I used two products from the suite, Norton Disk Doctor (NDD), to isolate the fault, and then Disk Editor, to fix it. I always find working in a plain environment (outside of Windows) the best for this kind of work.

The first thing I did was run NDD over the disk. It read the DOS boot record (or master boot record - MBR) fine, but when it read the first FAT it said they were different (it failed to read the first one, read the second one and compared), and offered to fix. I said NO, don't fix it - keep testing the disk. It did this and found no other errors. So I knew then I was dealing purely with a dead FAT #1.

The key point here (aside from isolating the fault) is that I didn't write to the faulty disk at all. I could have done this whole thing with the write-protect tab on (although perhaps foolishly, I didn't).

So, the next step was to make a good disk. This is because the old disk may be physically damaged and I don't want to try and fix it, I want to get the data off it and then throw it away.

I first formatted a new disk. Then I went into Disk Editor, and copied all the good sectors to my hard disk. Then I copied them back to the disk I just formatted.

Disk Editor can copy "through" an error. So you can tell it to copy every sector on the disk, including any damaged ones, and it will make a file on your hard disk the exact size of the floppy. Any unreadable sectors will be "zeroed out" (replaced with 0's).

So I told Disk Editor to copy every sector from 0 to 2879 (the end of the disk) to a file on my hard disk - and it did, filling the area where the unreadable sector was (sector 1, the first sector of FAT1) with zeros.

I then inserted my freshly formatted disk, opened the file with Disk Editor, and wrote it over the top of sectors 0 to 2879 on the floppy disk.

I was left with a good disk with a missing sector 1 (the first sector of FAT1). Since there is a spare copy of the FAT, FAT2 starting at sector 10, and it was copied along with the rest of the good sectors, I then went to it (again, using Disk Editor) and saved sector 10 as a file onto my hard disk. Finally, I opened this file with Disk Editor and saved it over the top of sector 1 on the good floppy disk.

I then quit to DOS and was able to copy all the files off the disk, intact, using a standard DOS COPY command.

Ta-da! :)

Note: If sectors 1-4 had been damaged, rather than simply sector 1, I would have copied sectors 10-14, rather than simply sector 10, back over sectors 1-4.

Note: If sector 0 (MBR) had been damaged instead, I would have simply formatted a new disk, and dumped sectors 1-2879 from the bad disk to the newly formatted disk. I wouldn't need to touch sector 0 on the good disk because it's just been formatted. On a floppy disk, all MBRs are alike.

Careful! If Disk Editor is looking at your hard disk instead of the floppy, you could save over your hard disk's MBR, FAT1, FAT2, and its first 2879 sectors. Pay attention! Ensure the floppy disk drive is selected before you write any sectors to disk!

DOS/Windows 1.44Mb floppy disk usage
sector rangeusage
0boot sector
1-9 FAT1
19-32root directory
33-2879data area
data courtesy Norton Utilities help