Sorting out termination issues with 50-68 pin (or SCA) adapters can be a nightmare. A good adapter needs to provide active termination of the 8 data pins not used on the drive side (not all do), and you'll also need to properly terminate the entire chain at the last drive/adapter on the chain. I also *believe* that Macs require one *and only one* drive to supply termination power (memory's a bit rusty on that one), and not all adapter-u320 drives combinations can do that. (If the chain isn't terminated correctly that might explain why your CD drive doesn't work correctly without the onboard-terminated original drive installed.)

(Oh, and aaaand one more thing: Note that for u320 drives to work they have to support dual cabling modes. If the drives *only* work with u320 LVDS cabling then they won't work no matter what you do.)