I have a 200GB 7200RPM DiamondMax 10 Maxtor hard drive (Model 6L200R0) in an IDE-to-FireWire external enclosure (some generic one, model ME-320 IDE). This enclosure's status lights are green and red. Green for power and that everything appears "good," and red for read/write and if there is an error. The Maxtor HD one day caused the enclosure to have a solid red light upon powering it up (when this happened before, it was a faulty IDE cable, which has since been replaced), and the drive would not mount, nor would a utility see it. Other drives work in this enclosure.
The fix was simple enough. Naturally the hard drive was set to a "Master" device, as it is the only device on that IDE bus. I changed the jumper to "Cable Select" and voila, the red light went away and the hard drive mounted! The drive seems to be fine, no bad noises or anything, but I still promptly copied off the data to another drive because of paranoia.
[robg adds: I can't test this one myself, obviously, and it doesn't make much logical sense to me. But if it helps someone else...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061106181229173