If you set PRI=0 into massive icon, you will get a transfer rate of 11KB/s and you can move the mouse...
If you set PRI=1, you got 24KB/s but the mouse freeze during transfer...
(don't try higher values, it gives the same rate... on 7.14MHz 68000 based Amiga)
@Cammy, PFS is fine but totally unusable between Mac/PC and true Amiga.
The best is 2GB thumb formatted in FAT32 and fat95 installed in the amiga system (put fat95 in l: )
I don't know why 4GB thumbs need to be validated each time they're plugged... so I think 2GB is the upper limit.
If you are curious, give a look at ANAIIS gui, interfaces menu (there are clickable trees, done with intuition...)