Mac O/S wouldn't recognize a 32 MB ProDOS Partition on a CF card

[AppleCPM <a2@wilserv.com>]

Hi, y'all!

   I move various CF cards between a IIe and a Mac 520c.  The IIe is
equipped with a SCSI card connected to a device that accepts CF cards
inserted into a PCMCIA adapter.  The 520c runs the "IIe" emulator and
can accept the PCMCIA adapter.  Among my collection of CF cards is one
with a 32 MB capacity that has one 32 MB partition that I use to boot
the IIe.  I inserted the PCMCIA adapter containing that 32 MB CF card
into the 520c and was shocked to see that I was being asked if I
wanted to format the CF card.  The shock wasn't too bad because I had
a backup of the CF card.  I was relieved to find that the IIe found
the partition on the CF card to be OK.  So, why did the O/S on the
520c not recognize the CF card?  This problem was solved by storing
binary zeros in to the last two bytes of the first block of the CF
card.  Those two bytes were originally $55AA.  I'm guessing that two
bytes had that value because the CF card was originally formatted as
an MS/DOS volume and that whatever program I used to format the CF
card as a ProDOS volume left those two bytes unchanged.

Willi