I have an Adaptec 2930 SCSI card which I bought specifically to use with a SCSI ORB drive. When I started using X, I could not get the ORB drive to work. A call to Adaptec solved the problem.
The tech support person told me to remove the Adaptec driver from System/Library/Extensions. The ORB drive has worked fine ever since. The name of the driver to be removed is something like Adaptec2906-30.kext.
I don't know if this strategy will work for other Adaptec cards and an ORB drive. Tech support made sure to ask me if that was the only SCSI device I had, so this may not work if you have other SCSI devices.
[Editor's note: Untested here; I have the SCSI card, but no ORB drive!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020328200225222