Ways to implement a selectable 3rd drive

[admin]

For a 5.25" disk drive you can expect to need 350mA for the 5v side and about 1A for the 12v side. These are the peak currently, and the 12v side will vary depending on how tight the disk is in the sleeve. What you could do is use 2 of the 3 extra wire for 12v. The 3rd wire is 5v. There are numerous grounds on the 34 pin ribbon cable, so you don't actually need an extra ground.