Big storage companies stopped recommending RAID 5 a couple of years ago. But I still see small 4-drive arrays touting RAID 5 for home and small office use.
Big mistake. You want to save money, but you also want to keep your data. RAID 5 isn't worth it.
What's the problem?
The problem is that RAID 5 only protects against a single disk failure. But SATA drives are spec'd at one Unrecoverable Read Error (URE) every ~12.5 TB.
Let's do the math.
In a small 4 drive array using 2 TB disks, if you lose a disk you have 6 TB - 3 drives - of remaining capacity. That includes the parity data used to reconstruct the data lost on the failed drive.
Reading through that 6 TB you have a better than 40% chance of encountering an URE - and at that point the disk rebuild will stop since the RAID controller doesn't have the information it needs to reconstruct your data.
Then you pull out your backup copies. You have backups, right?
How to use a small RAID array.
4-drive arrays have lots of advantages: cost; performance (with FireWire or eSATA) fast enough for HD video editing; and portability.
But if you care about your data, RAID 5 is too big a threat. And if you don't mind risking your data - as in performance driven apps like video editing where the data copies are on tape or another disk - RAID 0 (striping) is cheaper and faster.
Most small arrays come with a RAID 1 (mirroring) option that copies your data to 2 different disks. Lose 1 and the other should have it - subject to the occasional URE.
If you want availability and better performance use RAID 1+0 - often abbreviated RAID 10 - which combines mirroring and striping to provide 2 complete copies of your data with the performance of 2 striped drives.
The Storage Bits take
The attraction of RAID 5 is that it gives you 3 drives worth of capacity on a 4 drive array - but at the cost of having to use backups if an URE is encountered. Better to use RAID 1 and get 2/3rds the capacity of RAID 5 with a much lower chance of data loss.
The biggest storage mistake consumers make is to believe that any storage device is 100% safe. It isn't.
Maintain at least 2 copies of any data you value. If the data is vital, make that 3 copies. And if thinking about RAID levels makes your teeth ache, consider a Drobo or the new Drobo Pro.
Storage is cheap. Use lots.
Comments welcome, of course. Check out an earlier post Why RAID 5 stops working in 2009 for more details on the RAID 5 problem.