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[mechy]

Some things i have noted on the 4091... There are a commodore made one and dkb made ones, i haven't tested for any differences between them. 10MB/s is a theoretical maximum,theres no gaurantee it will run it exactly..Your best chance though is with:
A fast 10K rpm drive rated for fast scsiII that has ample buffers on it(no adapters),Latest 4091 roms,Active terminatiors throughout,Synchronous mode, shortest possible cable.Run SFS(smart file system) and SET the max transfer correct(big one here!).althought passive termination works on most scsi,it a hack at best and something more suited to old scsi 1 imho.
The 4091 having to dma to the slow motherboard ram because of the A3640 may come into play.
Now realize alot of factors can hamper all this goodness like different buster revisions,other scsi devices on the chain with broken firmware(crappy toshiba scsi cdrom drives come to mind),old scsi1 or non fast devices,bad cables,S-L-O-W FFS!

Theres a reason that SYSINFO is called sys(MIS)info.. try diskspeed4.2 or some such for more accurate results.

Scsi is almost always better on amiga compared to ide.Scsi off a good accelerator can be night and day.. one reason the warpengine,cyberstorms,etc do so well.