Quite frankly, SCSI drives still have a higher mean time between failure (MTBF) than ATA drives if you look at the vendors' specifications.
Although this gap is closing as the MTBF of many of the vendors ATA drives continue to increase, we would still expect the SCSI drives to have a lower failure rate in a heavily-utilised NAS.
So if reliability is critical and you expect the NAS to take a pounding, then SCSI would be a safer bet. If, however, the load on your NAS is not too high or the thought of replacing a drive occasionally when they fail does not worry you then ATA drives would do nicely.
Probably not, so you should seriously consider redundancy, and this does not begin and end with redundant power supplies. Other redundant features can include LAN and Fibre Channel ports, disk controllers, and in some cases the entire NAS controller is duplicated.
Generally if the NAS does have duplicated ports or controllers these can be configured to load balance, further improving the unit's performance when it's in its peak of health.
