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:
“How To Create Disk Image on Mac OS X With dd Command” (Vivek Gite, August 2018)
“Running dd. Why resource is busy?” (StackExchange, March 2016)
“How to mount raw disk images?” (StackExchange, May 2015)
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.
diskutil list to find the identifier of your external disk drive.
It will be marked
(external, physical). In my case, what I see is
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,
/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.
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.ddinstead, 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
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
because it should still be unmounted from when you ran
diskutil umount /dev/disk3
To mount a
dd disk image on OS X
This one is simple. Run
cp mydisk.dd mydisk.dmg, and
mydisk.dmg in Finder. The mounted volume will magically appear
/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
.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.