The Koolance drive coolers are Aluminum (*NOT* annodized!) and use 1/4" ID tubing. Doesn't sound nice until you consider that hard drives don't chew as much juice as CPUs (well, until you get a LOT of them). In this case, 1/4" ID is fine.
So I have a bunch of the Koolance blocks on a dedicated aluminum loop. The radiator for that loop is a Tru-Cool transmision cooler (a BIG one, not the small one like the Swiftech one). It's just using a smaller maxijet for the pump.
Koolance has "2nd hard drive kits" that they sell, but they are only usefull if you have a Koolance case. I don't have a Koolance case, but rather a large black Cube case (A9891). So they would be fairly useless to me. Besides, you can make one yourself with spare metal and a drill, maybe cost a buck. You can fit 2 drives on a single block this way.
I have 7 (6 in use, one waiting for next drive to show up) currently. Any more, and I'll probably need an additional pump.
I don't mix the loops between CPUs and drives. They are entirely seperate. If I had Copper Drive blocks with 1/2" insides, yeah, I'd mix'em, but they aren't so I don't.