The ARS-2000SUP has different firmware than the ARS-2000SU. I had a problem where my ARS-2000SU units would not auto boot on m68k Macs or on Amigas when they had the ARS-2000SUP firmware. I downloaded the latest ARS-2000SU (no P) firmware from the Acard site and flashed it (had to install Windows on a spare motherboard to do it... what a pain!), and everything auto booted just fine after that. I bet your issue is the same.

Separately, there are instances where the Mac SCSI driver which gets installed onto the drive isn't the right one. You may have both problems, but I'd recommend fixing the firmware problem before looking at the SCSI driver problem.