I can't read this floppy in one Mac, but it works just fine in
my other Mac.
There are several possible culprits, the most likely being the
disk format. The earliest Macs used single-sided 400k floppies,
which some of the Power Macs cannot read. In 1986 Apple introduced
double-sided 800k floppies on the Plus. Previous Macs will not work
properly with double-sided disks.
A few years later Apple started shipping high-density floppies
(called FDHD or SuperDrives). These 1.4 MB disks cannot be read in
any Mac without a high-density drive. If you're trying to read 1.4
MB disk in a Plus, unupgraded
Mac II, or early SE, that could be the problem.
Another possibility is that you've managed to format a
high-density (HD) floppy in an 800k drive. I've seen this fairly
ofter. Macs recognize HD disks by the second square cutout on the
disk. Since pre-HD Macs don't look for that, they are perfectly
content to format these disks to 800k. To make these disks readable
in a newer Mac, put opaque tape or a label over the HD square.
Still another possibility is that you are using IBM formatted
disks. Macs with HD drives can read and write IBM floppies,
but only if PC Exchange, DOS Mounter, or a similar program is
installed. But that's only in Macs with HD floppies - you can't
use IBM format disks in 800k drives.
With modern Mac lacking an internal floppy drive, reading Mac
and PC high density drives isn't a problem, so long as you have a
USB floppy drive. However, none of the USB floppy drives will read
Mac 800k floppies.
Finally, there's the possibility that one of your drives is out
of alignment or needs to have its heads cleaned. Cleaning is
usually simple: insert a cleaning disk. And sometimes blow out the
dust bunnies with compressed air - it's amazing how much dust can
accumulate inside a floppy drive.
The older a drive and the more heavily used, the more likely
misalignment. The only solution is to replace the drive, which is
typically about $60 and usually an easy installation. (Big
exception: compact Macs such as the Plus, SE, and Classic.)
Further Reading
