Some Apple IIGS users have discovered that Disk II drives do not work
properly when the controller card is installed in slot 1, 2, or 3. On
startup in this configuration you will get an "UNABLE TO LOAD ProDOS"
error message.
The reason is that there is some special hardware that was designed into
the Apple IIGS just to take care of handling Disk II access. The hardware
recognizes Disk IIs, and when an access is attempted, the system switches
into slow mode on recognition of the Motor Start signal.
This causes the "UNABLE TO LOAD ProDOS" error, since the system cannot
switch speeds quickly enough to begin reading the beginning of the boot
blocks on the disk. This is one of the time-critical functions of ProDOS,
and it does not function correctly. The problem occurs only when using
Disk IIs and only when in the lower numbered slot 1, 2, or 3. This follows
the Pascal convention of no bootable devices lower than slot 4.
Here is a workaround:
If the system is running in Slow mode when the access to the Disk II is
performed, the boot blocks are read correctly, and the system starts up.
The same is true for cataloging the disks.
If you can run your program in slow mode, then you can use the Disk IIs in
slot 1, 2, or 3. If not, then you must install the Disk II in slot 4, 5, 6,
or 7.