so you want two hd's at 250 gigs a piece, and then you want to replicate the two or no? I don't really see what you mean when the first disk is full, then filling up the 2nd HD..! Please elaborate a bit more
is Raid 0 out of the question? If so, you can just mount the second HD and use it just fine, if you don't care about HD security/integrity when 1 of the 2 HD's goes out It's just a matter of time in my opinion!