Thanks, that clinches it: the Optiarc is the drive mechanism, not the software. If it came with the iMac as the default optical drive, Mac OS X should have the driver for it and things should work OK with that. Still, there may be a newer driver available; check Apple's driver updates or look for updates at the OEM's web site. It's also quite possible that the drive is not functioning properly, but that's difficult to ascertain without obvious problems.
As to firmware, the Optiarc RW AD-5670S featured in
this Firmware Page forum thread, which mentions the presence of
a newer firmware version (2AHI). No idea if that's helpful or even advisable, or if this is even relevant to your Mac and drive model.
