Three year warranty
A five year warranty would run the risk of cutting the legs out from under the nice secondary market that is the backup of the the backup. A three year warranty makes sure that enough people are nervous enough to buy two sets, in case one breaks.
RAID1 despite costing you 50% of your storage space doesn't quite offer the protection in this configuration that many assume. Essentially you have two identical disks coming off the assembly line around the same time, being subjected to identical environmental conditions, and similar workloads. The chances of them both failing around the same time are higher than for non-identical disks. RAID1 only offers protection for one disk failing. If both go you're scuppered. The only way to avoid this happening is to open up the new device and hit one of the drives with a hammer/luser hard enough that it will fail first by a safe margin, but not hard enough to break it! Most people prefer not to do this.
So you need a backup of the backup. Hence the three year warranty to underline that requirement.
That being said, I'd still prefer five years :)