Apple shipped several versions of external 3.5" drives: the beige
single-sided 400k original Lisa/Mac drive; the beige Mac-only 800k drive; the
white IIe/c-only (IIgs, too, but slow data rate) 800k UniDisk 3.5 (with
onboard 65C02 chip); the platinum 800k Apple 3.5 for use on Macs and Apple
IIgs & IIc+; the 1440k platinum Apple FDHD SuperDrive for use on FDHD Macs
and IIgs/IIe's with the SuperDrive controller card.
The Apple external SuperDrive (FDHD) also has the capability of being used as
an 800k drive, and normally requires the Apple "SuperDrive" controller card
to enable reliable data transfer on a "slow" Apple IIe. As I understand, it
works in 800k mode with a UDC (Universal Drive Controller card from a 3rd
party.)
The Apple FDHD drive and card are optimized to transfer data to/from a
diskette initialized (formatted) at an interleave of 2:1 (every other block
on a track is sequential) except on a IIe. The IIe's CPU is so slow that the
optimal interleave for 3.5 diskettes initialized on it is 4:1 (every fourth
block on the track is sequential).
When one has an Apple UniDisk 3.5 (white color) and its controller card
attatched to a IIe/c and uses the Apple IIe/c System Utilities to initialize
(format) a 3.5" 800k diskette (Copy II Plus, also), it sets the interleave to
4:1. This UniDisk 3.5 has a 65C02 microprocessor on board to handle the slow
data transfer rate between the drive/card and the IIe/c's system bus. If you
then use a diskette initialized at 2:1 (on a Mac with the ProDOS extension or
a IIgs) on this setup, the data transfer rate is extremely slow as the
diskette has to spin twice around to get to the next sequential block.
The Apple IIgs and Mac have fast enough CPUs so that they can read data on
diskettes that are formatted at a 2:1 interleave. That means that they can
assimilate every other block as the diskette rotates at its normal speed. The
IIe/c have slow CPUs, so they need to work with diskettes formatted at 4:1
interleave; they can only assimilate every fourth block of data as the
diskette spins at its normal speed; if a 2:1 diskette is used, the next block
of data is not on the fourth block, as expected, but all the way around
again; this makes for very slow data transfer.
In general, with the Apple IIe, one should stick with these combinations:
white UniDisk 3.5 and its own controller card; platinum 800k Apple 3.5 (or
clone 3.5 drive) and a UDC card; platinum Apple FDHD SuperDive and its own
controller card.
With the original Apple IIc that has the ROM chip upgrade, the only drive
that will work is the white Unidisk 3.5. With the Apple IIc Plus, you can use
either the UniDisk 3.5 or the platinum 800k Apple 3.5. With the Apple IIgs,
you can plug into the built-in disk port the white UniDisk 3.5 (expect slow
transfer rates) and the platinum 800k Apple 3.5 drive; to use the Apple FDHD
superdrive in 1440k mode, you need to connect it to its own controller card
and set that slot to "Your Card".
