Thx, but meanwhile I found a pretty easy solution
After patching from R2.2 to R3.2, firewire support was added
Overall the drives behavior changed:
Before: Disconnected from USB/FW, the drive tried to read a CD/DVD... Even with no media in the drive.
After: Everything's fine. No more "blind" reading, no more problems with Firewire.
Anyway thanks to everyone