Here's my current setup

EVGA 780i
EVGA 8800GTS OC
Intel Core2 Quad q9450
One DVD Drive (hardly use it at all)
1 250gig WD HDD
Thermaltake Toughpower 600W psu


Basically, my external drive enclosures PSU went out and i need at least 3 of the the drives back up ASAP. Can i add 3 more WD 250gig HDD's and still be safe on power with my current setup?

oh yeah, no prob for that ! they don't draw much about 12 each on startup for an average drive and 7 or so runnin.

Yeah.

My primary PC has a Seasonic S12 550 PSU which powers the following:

Q9450
X1900XT 512MB
2 DVD Drives
4 Hard Drives

A typical hard drives uses about 12w - 14w under full load, and about 5w or 6w idling.

Hard drives are entirely unlike CPUs and video cards when it comes to power draw. They never get as high as 20w when spinning at full speed while writing, (i.e, "full load," ) and don't run at full power as much; if the rest of the parts, like the CPU and GPU, are at full load, then the HDDs have to be idling otherwise they'd be severely bottlenecking the other parts. So yeah, you'll have no problem.