I just bought an el cheapo (coolmax HD-211, 2.5 inch external USB 2 to be exact) enclosure. The drive wouldn't spin up set to master. I could just barely connect with a jumper attached because of the position of LED lights on the enclosure's circuit board. Set to slave it would spin up and then nothing, and with it set to "cable select" it spins up and the Mac (12" powerbook with USB2 ports, OS X.4.7) gives me a message to the effect that I have a choice of formatting the drive or ignoring it (unfamiliar format or unable to mount or something like that).
My question, before I return the enclosure, is: is it normal to have to reformat a drive when used in an external enclosure after it's been formatted and used as an internal? I've used this drive for a while with OS X internally, and it works fine that way. I don't want to format it because I have a bunch of stuff on there already.