most 3.5" desktop hdds use both 12v & 5v; for the spindle motor (and maybe for the voice coil of the heads), 12v power line is used...i think to be less demanding in terms of current.
5v line is used mainly for the logic circuit
if you go to the hdd maker's websites, you will find specification pages where they will tell you exactly how much they are asking you to provide to the hdd for the stable operation.
i think 3.5" uses 300 to 500mA from the 12v when idle & at the heavy operation it will go up to around or even more than 1.0A; i think 250 to 500mA for 5v is enough
the problem is when the hdd starts up; because motor produce highest torque when it is not moving, starting up current might be as high as 2.0A...maybe even more
for the 2.5" notebook type uses only 5v for both motor & the circuit...the current it nees is probably less than 1A even during the startup
if you use 2.5" notebook type 5200rpm drive, i think the pstwo power supply can provide enough power...just add 5v voltage regulator on between 2.5" hdd & ps2 ac adaptor
the best way to add hdd to pstwo imho is to use external usb/ieee1394a adaptor; regular pc ide cable & connect to both usb2 to ata adaptor & pstwo, then put a power switch between usb2 adaptor & power supply so that you can connect your ps2 to pc without taking apart
Ref +: psaras, marcus21, sirusxXx, gamecubesuxs