RAIDs are designed to prevent the loss of data in the event of a disk failure - but the risk to the data in a RAID set is dramatically increased while you're rebuilding the RAID set with a replacement disk, as Dave from Netapp explains. Although this is the case with all RAID 5 setups, this applies to an even greater extent to ATA/IDE/SATA based RAIDs, due to the higher change of failure in the cheaper, more consumer-grade drives.

Anil Gupta over at NetworkStorage follows up with some more information on the same subject.