It may be as simple as going into the Adaptec BIOS and changing the speed, but most likely not. Adaptec BIOSes generally detect the correct speed for that configuration.
GOSHARKS hit the possible issues on the head. Make sure you are using a good brand and quality LVD Ultra 160 or Ultra 320 cable (A.k.a Ultra3 or Fast-80 for Ultra 160 or Fast-160 for Ultra 320. SPI-3) and a good quality LVD active terminator.
Make sure it is plugged into the internal LVD SCSI connector and not the internal Ultra Wide connector (both 68-pin). And as GOSHARKS said, make sure that the drive is not jumpered in SE mode.
For optimal performance one should use the shortest possible cable that will reach.
Just cause I feel like it, general SCSI rules that don't apply to your problem:
Start with SCSI Id 0 for hard drives and go up. CD-ROMs and other devices should start at SCSI Id 6 and go down. If you have more than 7 devices other than the SCSI controller, put your boot HD on SCSI Id 0. Then start the rest of the HDs on SCSI Id 8 and go up to 15 from there.
The above will provide the optimal performance based on SCSI Id priorities, slower devices need a higher priority otherwise they will often fail to negotiate for the bus.
Avoid mixing cable types (generally mixes impedances) as it adds reflections (noise) to the bus.
The SCSI-2 specification states that no more than four devices may provide termination power (TERMPWR) and that the host adapter must provide termination power. I generally just let the host adapter provide termination power, but theoretically if you let the last device on each end of the bus provide TERMPWR you may increase the speed of the bus. SCA devices generally cannot provide TERMPWR. The more devices that provide TERMPWR the hotter the TERMPWR wire gets and the more current used (more power wasted). More than four devices providing TERMPWR may (and most likely will) cause excessive heat on the TERMPWR wire.
The SCSI bus must be terminated on both ends.
The SCSI bus uses an active low signal pulled high by the terminators using TERMPWR. A device asserts a 1 on a wire by pulling it to ground and then lets go (High-Z) and lets the terminators return it to 0 (Ideal: 2.85V, SCSI-2 minimum 2.0V).
The terminators are a lot more important than is immediately obvious as they determine the rise time of the signals on the bus.
There you go, more than you ever wanted to know about how to get the best performance out of your SCSI bus.
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