This article is from the Macintosh hardware FAQ, by Elliotte Rusty Harold firstname.lastname@example.org with numerous contributions by others.
There have been three kinds of floppy disks in the history of
the Macintosh. The original 128K Thin Mac (which used to be called
a classic Mac before the advent of the much superior Mac Classic)
and the subsequent 512K "Fat Mac" used 400K, single-sided
double-density diskettes. These disks are outdated, and it's highly
unlikely you'll actually see any. If you need to exchange data
with an older Macintosh, you'll need to use disks formatted as
single-sided. Since very few, if any, stores still sell one-sided
3.5 inch disks anymore, it's fortunate that all Macs deal quite
happily with double-sided disks formatted as single-sided. Just
click the button labeled "One-sided" after you select "Erase Disk"
from the Special menu.
*Neat Trick alert* Sometimes disks that fail formatting as
double-sided can be formatted as single-sided. Even neater trick:
In System 6 the shareware init BAD can map out bad sectors on a
floppy disk which lets about 70% of bad disks be formatted. (System
7 does this automatically.) See
Neatest trick of all: All name-brand diskettes (SONY, Maxell, etc.)
come with lifetime warranties. A lot of offices keep a bad disk box
for everyone to dump their bad disks in and send the disks in for
replacement when they collect ten or so; but it's been my experience
that if you return just a single bad disk these companies will send
a whole ten pack as a replacement.
With the introduction of the Mac Plus in 1986, Apple also
introduced a larger disk drive capable of reading and writing 800K,
"Double-Sided Double-Density" disks, DSDD for short. The only way
to tell these disks from the earlier, one-sided diskettes, is by
the label on the metal cover. Unformatted these are identical to
the 720K disks common in the IBM world. With the Mac IIx Apple
introduced what's alternately known as the Superdrive or "FDHD,"
short for "Floppy Drive, High Density." The FDHD (pronounced
Fud-Hud) can read and write all of the previous kinds of diskettes
plus double-sided high-density disks which are distinguished by two
holes in the disk case rather than the normal one. The FDHD uses
the extra hole to recognize a high-density disk.