If it's a bootable RAID 1 array, then it's using both drives, however it should be able to run off a single drive in degraded mode unless of course a virus messed up the files on those drives or you accidently deleted a needed system file. Remember, in RAID 1 whatever gets written to one drive gets writen to both. RAID is not a backup solution. It's only meant for redundancy in order to keep a system running if a drive fails.