The whole idea of RAID (Redundant Array of Independent Disks) is to allow for data redundancy (if one disk dies you dont loose the data). I have never played around with a proper raid controller, but i expect that yours is confused as to why you only have the one disk plugged into it.
Level 1 RAID basically replicates one of your hard drives onto another, so if one dies then you still have the other.
Level 0 RAID (not true RAID - but improves performance) stripes the data across both disks, so half on one, half on the other. But if one disk dies the you loose everything.
I would assume that your raid controller is guessing that one of your disks has died and is trying to recover something... but like i said i heve never played with one so i'd wait for someone elses opinion first.
Either way i would suggest chucking two disks on the controller then you can speed them up