I have two 250gig drives running all the time, one for image data and the other for the OS (Linux) and everything else.
I also have one 250gig drive that I only plug in when doing backups of my images, the rest of the time it just sits in the case unplugged, yeah I know if there is a fire I lose the lot, but if my house burns down I think that would be last thing I will be worried about
And transfering data from one sata2 hdd to another is wayyyyyy faster than via usb.
Cheers David