raid5 does redundant storage. Replacing drive #2 should prompt the system to rebuild the lost data that was on drive 2 onto the replacement drive.
Normally a setup like that would default to that +1 as a hotswap, and rebuild the data onto that drive and tell you drive 2 is gone, so you could replace that and it would become the hotswap... sounds like it wasnt configured properly.
The data should all be there, however... just inaccesable until drive 2 is replaced and the data rebuilt.