The Rev A blue and whites have a defective IDE controller which not only doesn't like second drives, it doesn't like pretty much any non-factory drive. If you attach a recent drive, you'll get corruption problems.
Rev A's have a flat bracket for the hard drive, no way to attach another drive. Rev B's have a U-shaped bracket designed for two drives, as well as a drive cable with sufficient connectors for 2 drives.
Rev A: 300, 350, 400MHz
Rev B: 350, 400, 450MHz
tooki