Making floppy disk images under OS X

This week I got an external floppy drive, dug up some old 3.5” floppy disks from my youth, and imaged them to see what was on them. Here’s the process I used.

Thanks to these articles for getting me started:

To make a disk image of a floppy disk on OS X

Insert the floppy into your external disk drive. Wait for it to auto-mount.

Run diskutil list to find the identifier of your external disk drive. It will be marked (external, physical). In my case, what I see is /dev/disk3.

Run diskutil umount /dev/disk3. You’ll see the message “Volume (whatever) on disk3 unmounted”.

This next step can be fatal if you make a typo! Carefully, run sudo dd if=/dev/disk3 of=mydisk.dd bs=512 conv=noerror,sync, replacing /dev/disk3 with the identifier of your external disk drive. The argument conv=noerror,sync is not strictly needed, but if you omit it, any single I/O error on the disk will abort the transfer and leave you with an incomplete image. The argument bs=512 gives you a “block size” of 512 bytes, the same size as one of the sectors on a 1.4MB floppy.

Be careful with dd! If you type of=mydisk.dd instead, you’ll trash your floppy disk. If you type if=/dev/thewrongdisk, you’ll copy the wrong disk (and it might be very big).

If all goes well, you’ll see output almost identical to the following:

2880+0 records in
2880+0 records out
1474560 bytes transferred in 114.128214 secs (12920 bytes/sec)


But, if the disk had I/O errors, you’ll see output like this:

dd: /dev/disk2: Input/output error
2872+0 records in
2872+0 records out
1470464 bytes transferred in 113.303469 secs (12978 bytes/sec)
dd: /dev/disk2: Input/output error
dd: /dev/disk2: Input/output error
2873+0 records in
2873+0 records out
1470976 bytes transferred in 114.298722 secs (12870 bytes/sec)
dd: /dev/disk2: Input/output error
2880+0 records in
2880+0 records out
1474560 bytes transferred in 114.299519 secs (12901 bytes/sec)


Each time it sees an I/O error, dd will skip to the next 512-byte sector, padding the current one out with zeros. (Notice how 1470464 bytes is evenly divisible by 512.)

When you’re done, push the button to manually eject the floppy disk from the disk drive. There should be no need to unmount it via Finder or diskutil umount, because it should still be unmounted from when you ran diskutil umount /dev/disk3 earlier.

To mount a dd disk image on OS X

This one is simple. Run cp mydisk.dd mydisk.dmg, and then double-click mydisk.dmg in Finder. The mounted volume will magically appear under /Volumes/volume-label (or apparently as /Volumes/Untitled if the volume has no label).

Beware — when OS X mounts a volume from a .dmg, it doesn’t make a “scratch copy” anywhere. If you create, modify, or delete a file on the mounted volume, you will change the structure of the filesystem in the original .dmg file!

Naming the file with a .dd extension (unrecognized by OS X) forces you to make a .dmg copy before you’ll be able to get at the files; which thus prevents you from losing data by accidentally double-clicking the original file.

Posted 2019-07-26