The bulk only with transparent scsi commands (8,5,80) is easy to implement, after having understood what to do with 2 endpoints (in and out) and the 3 phases Command Block Wrapper (CBW), DATA, Command Status Wrapper (CSW) and has nothing to see with usb 2.0 or not.
Perhaps massstorage on Sirion is mixed with very old CBI implementation, which is a horror to code, maintain and fix.
I'ts bad to see that OS4 is not reliable.
FAT32 4GB keys are usable on my A2000 68000/Highway/Subway Workbench 3.1 with fat95.
Big shame.