Date   : Thu, 04 Jul 1985108:19:00-MDT
From   : Andy Brown <brown@CMU-CS-K.ARPA>
Subject: using both side of disks

Whether it is a good idea to use both sides of a floppy disk depends
on two things:

1)  Is the magnetic material on both sides of the disk error-free.

2)  Are the disks designed to be "flipped".  That is, can the disk
    safely rotate in both directions with no damage.

Most floppy disks are manufactured with a magnetic coating on *both*
sides.  The disks are then tested to make sure that there are no
defects which would make them unsuitable for data storage.  If one
side of a disk is bad, then it is usually ceritfied as a single-sided
disk, if both sides are good, then it is certified as a double-sided
disk.  When you buy blank disks, it will always say on the box whether
the disks are single or double sided.  Therefore, most single-sided
disks have *known* defects on the uncertified side (you can tell which
side is the correct side because it has the label on it).

All floppy disks have what is known as a "liner".  This is inside the
cover of the disk and it has two main functions: to minimize friction
as the disk is spinning; and to catch and redirect any dirt and dust
which may have found its way into the disk jacket.  There are a wide
variety of liner designs, and some of them are intended for use with
disks that will only be spinning in *one* direction.  Flipping a disk
with such a liner could cause dirt and dust that was trapped by the
liner to be released onto the disk, creating the potential for head or
disk damage.

It must be mentioned that there are really *two* kinds of double-sided
disks.  Some disk drives, (such as the double sided drives in the IBM
PC) have two heads, one for each side of the disk.  Thus they use both
sides of the disk, but the disks always spin in the same direction.
When you "flip" a disk on a single-sided disk drive, such as the
Commodore 1541, the disk will spin in the opposite direction when you
flip it.  Thus there are some floppy-disks on which both sides can be
used, but you might not want to use the *back* side on your Commodore
1541 disk drive because using that side will cause the disk to spin in
the wrong direction.

It is absolutely safe to use both sides of a disk on a 1541 disk-drive
if the disks are certified double-sided on the box, and there is a
write-protect notch cut on both side edges of the disk.  If not, then
it is risky to use the uncertified side for data storage.

If you buy a commercial software product which uses both sides of a
disk, then it is probably safe to assume that they are using the
correct kind of disk.


--- Andy Brown