I'm not an expert, but I offer an opinion.
AFAIK, large USB drives are formatted using NTFS (sp?).
Even PC's can only read large USB drives with NTFS, not Fat95.
To read (not write, unfortunately) you can use GiggleDisk from Aminet.
Having said that, I did repartition my 80gb drive into 30gb with Fat95 and 50gb with NTFS, and both the miggy and pc can read all the 30gb. But, it was formatted using Partition Magic so maybe that gives a clue?