What I did on my A1200 with a 20G 2.5" IDE was to make a standard 2G FFS partition at the very start of the drive. This partition can be read by the stock 3.1 ROM in the A1200 no trouble. The OS then loads the FS and patched IDE driver from 3.5 so that other partitions (and larger partitions) can be read/written after booting. That works fine (for me).
