Ex-G5 hard drive not mounting in external case

So my several year-old g5 died, and I've bought a new Mac Mini and planned on taking the hard drives that i had out of the g5 and put them in an external case to use with the Mini. As you can all guess by my presence here, its all gone pear-shaped.

I had two hard drives in the g5, a 160g that came with it, and a 500g one that i added. I purchased a dual 3.5" HDD external enclosure, and put both hard drives in. When it's plugged in to the recently arrived mini, it simply doesn't show up, on the desktop or in disk utility.

Any help would be greatly appreciated.