certain brands of internal mechanisms in an external may not be Mac-bootable. I'd check the database at
http://forums.xlr8yourmac.com/drived....drivedb.lasso for internal mech. brands that are bootable, and making sure that you get an external that is bootable.
Especially Beware Of LaCie. They use a different internal mech. every week and you never know what you are gonna get.