How can I diagnose a superdrive that won't read disks? My MacBook (Snow Leopard) seems to be unable to read DVDs and CDs. When I insert a disk (DVD or CD, I've tried a couple of each), the drive spins up, and (it sounds like) moves the laser back and forth a few times, then ejects the disk. I haven't tried blank media.

I've searched around the nets, and it looks like I may be experiencing a relatively common problem, but I don't want to jump to conclusions. Unfortunately, all the threads I've found are filled with "me too, me too", and not a lot in terms of actual help. 

What tools can I use, or tests can I run, to try to diagnose (and perhaps even solve) the problem myself before I give up and replace the drive?
Try running the Apple Hardware Test.  If I remember right, reboot holding D.  If that doesn't work, reboot while holding D, with an OS X disk in.  Of course you'll need to have an external DVD drive for that to work.