Originally Posted by Wikipedia
In RAID 1 (mirroring without parity or striping), data is written identically to multiple disks (a "mirrored set"). Although many implementations create sets of 2 disks, sets may contain 3 or more disks. Array provides fault tolerance from disk errors or failures and continues to operate as long as at least one drive in the mirrored set is functioning. With appropriate operating system support, there can be increased read performance, and only a minimal write performance reduction.
One of the drives is creating errors or it's just having problems. The OS is working because the array can work with either drive or just one drive.
Remove the power connector from one of the drives and then test the remaining drive. If it passes then remove it's power connector, plug the other drive back in and test it.
The Raid Controller has it's own bios, when you enter the Raid bios it should tell you the status of each individual drive.
I would go ahead and buy another identical drive so that it's on hand as a spare.