I spent hours trying to figure out why my Spressa CRX-145S CD-RW, connected to an Adaptec 2609 card on my G4 running 10.1.5, was not being seen by the system, when everything worked fine on System 9.2.2. I tried all of the software fixes on the net, and nothing worked (except once after all of the software fixes listed below, until I rebooted).
Turns out the problem was that I didn't have a SCSI terminator on the open port of the CRX-145S! I didn't need one with 9.2.2, but apparently OS X is pickier about this. I snapped one on and now the system sees it, and I'm happily burning away!
Of course, you might also need to do some of the software fixes that I found listed here on macosxhints, summarized in jonsaw's comment (near the bottom of the comments) to this hint. I did all of the following (but try the SCSI terminator first, if applicable):
% sudo rm /System/Library/Extensions.mkextAfter that, restart.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021007054314293