ZDNet argues RAID 5 will reach the end of its useful life in 2009 because increasing hard disk capacity will make it almost certain that you'll run into an error that will make data unrecoverable on failed disks:
SATA drives are commonly specified with an unrecoverable read error rate (URE) of 10^14. Which means that once every 100,000,000,000,000 bits, the disk will very politely tell you that, so sorry, but I really, truly can’t read that sector back to you.
One hundred trillion bits is about 12 terabytes. Sound like a lot? Not in 2009.
Disk drive capacities double every 18-24 months. We have 1 TB drives now, and in 2009 we’ll have 2 TB drives.
With a 7 drive RAID 5 disk failure, you’ll have 6 remaining 2 TB drives. As the RAID controller is busily reading through those 6 disks to reconstruct the data from the failed drive, it is almost certain it will see an URE.
So the read fails. And when that happens, you are one unhappy camper. The message “we can’t read this RAID volume” travels up the chain of command until an error message is presented on the screen. 12 TB of your carefully protected - you thought! - data is gone. Oh, you didn’t back it up to tape? Bummer!