Well i too had heard that defragging didn't do much of anything under X, but not one to have blind faith, i installed a small OS X system on on my my external partitions, and booted from there to defrag my normal boot drive. It was pretty heavily fragmented, data thrown all over the place, and took quite a while to defrag... I rebooted and checked the disk, sure enough everything was degfragged and optimized, but you wouldn't know it from the speed. I couldn't see ANY speed increase, even minor, which really surprised me. I'm still anal enough to like knowing it's all cleaned up, but it's generally useless. Not worth the time and hassle, IMHO. But if you insist on trying it, you do obviously have to start up from another disk. Booting from another disk gives you a good opportunity to run a disk utility on it, i personally use norton disk doctor.
Good luck!
🙂
pnw