Hello,
I'm using a MyFlash (ADATA) 1 GB SD card.  The Zaurus says that it can't mount the card.  
I tried:
mkfs.vfat /dev/mmcda1 
(/dev/mmcda1:  No such device)
and
mkfs.ext2 /dev/mmcda1 
(mtfs.ext2:  No such device while trying to determine filesystem size).
When I try to mount the device:
mount /dev/mmcda1
(mount: /dev/mmcda1: unknown device)
No Luck!  I don't have an SD card reader, so I can't play around w/ this on my OS X computer.
Any suggestions?
Louis