This article is from the Introductory Macintosh FAQ, by Elliotte Rusty Harold email@example.com with numerous contributions by others.
Just as a floppy disk needs to be initialized before use, so a hard
disk must be formatted before it can hold data. You don't need to
reformat every three months; but when your system is crashing no
matter what you try, reformatting is the ultimate means of wiping the
slate clean. Reformatting your hard disk may even gain you a few
extra megabytes of space. Not all hard disks are created equal.
Some can hold more data than others. To facilitate mass production
and advertising without a lot of asterisks (* 81.3 megabytes is the
pre-formatted size. Actual formatted capacity may vary.) Apple
often formats drives to the lowest common denominator of drive
capacity. When you reformat there's no reason at all not to reclaim
whatever unused space Apple's left on your disk.
Unlike floppies hard disks need a special program to initialize them.
Most hard disks come with formatting software. Apple's disks and
System software ship with either HD SC Setup or Drive Setup, minimal
disk formatters which will format Apple brand hard drives *ONLY*. See
Most hard drive manufacturers ship appropriate formatting software
with their hard drives. Normally this is all you need to reformat
your hard disk. This software installs a "driver" onto the hard
disk. Most formatting software includes an option to update the
driver without reformatting the entire hard disk, and this can fix
some hard to diagnose problems without going to the trouble of
reformatting and restoring an entire hard disk. (Do backup before
updating a hard disk driver though, as a failed driver update can
leave a disk unusable.)
A number of general-purpose formatters are also available
which go beyond the bundled software to include features like
encryption, password protection, multiple partitioning, faster disk
access, System 7 compatibility, and even compression. Two of the
best are the payware Drive7 and Hard Disk Toolkit Personal Edition
($49 street for either). While there are one or two freeware
formatters available, none are likely to be superior to the
ones bundled with your hard disk.
PowerBook owners should be sure to turn off Sleep and
processor cycling before reformatting their hard drives no
matter what software they use. Otherwise disk corruption,
crashes, and data losses are likely.