RAID systems come in different levels, but the only one that truly lives up to the “redundant” part of the name is RAID 5. In this system, all the data is spread over all the drives in the device along with parity data. This means that if one drive fails, the data on the other drives can be used to reconstruct all its data using the parity information to fill in the missing parts. This allows the other drives to keep working. Replace the failed drive, and the remaining drives will reconstruct the missing data on the new drive.

The parity information for the other drives does take up storage space, so the total capacity of the NAS device is reduced by the size of one drive. In many cases, RAID 5 NAS devices come configured with four hard drives. For this piece, we used a 2 TB version of the TeraStation III from Buffalo Technology () that has an estimated street price of $1,300. This device is configured with four 0.5 TB hard drives. As a result, the total available capacity of the device is 1.5 TB when using the RAID 5 configuration.

RAID 5 makes data storage far more reliable, but it is not a replacement for backups. For example, if an important file is deleted by accident, it is deleted from all the drives. And if a burglar steals the NAS device, you've lost everything. The only way to recover data lost in either of these possibilities is to have backup copies. So don’t think you can skip the backups just because you have a RAID 5 system.

Note that there are other RAID levels that do not provide the same sort of protection. RAID 1 simply mirrors the data on two drives; when data is written to one, the same data is also written to the other. This is less sophisticated and does not have the error checking that the parity data can provide. RAID 0 is designed for speed, not protection. By striping--spreading a file’s data across multiple drives--and thus allowing the drives to retrieve their parts simultaneously, it can improve performance when working with large files. This actually increases risk, because more drives mean more likelihood of failure, and if you lose one drive, you lose the contents of all the drives.

RAID 5 thus gives you the best of RAID 1 and 0, since it offers redundancy for protection and the striping for improved performance. So for a business application, make sure you get a NAS device that has uses a RAID 5 configuration.