Maxtor Seagate certainly keep their power adapter specs secret! The only thing I could find was 12V, 2amps, tip positive power supply.
Theoretically 3.5" HDDs require 12 V @ 1-2 Amps for the motor, 5 V @ .5 Amps for the electronics. So the above spec makes sense.
If you mixed up the power blocks, that may help. If you need a power block for the unit, might call Seagate to see if you could purchase one from them.