To be even clearer: the really important thing is not accidentally to put 12v onto the 5v line. Hard disc power wiring carries both 12 and 5v on different wires, and if you're using pre-made cabling (like the cabling already inside your computer) all should be fine. It's only if you're building your own power cabling you really need to be careful not to put 12v in the wrong place, because then the magic smoke comes out.
That said, as
@LaPorta said, the SCSI2SD should just be powered by the SCSI bus most of the time, so you don't even need to connect a separate power cable to it to get it to work, just the SCSI ribbon cable.