I hope the top line (enterprise or RAID class disks) shoots up and the bottom line remains very low. If not, it shows the need for Qnap (and other NAS vendors) to explain the difference right below their recommended disk tables!
Yes, one of my friends worked that out whilst we were consuming fine ale one evening. As he pointed out, hard drive sizes have exploded over the very recent past but that UER hadn't reduced in proportion to disk size increasing and thus it's the statistical likelihood that needs to be considered. He thus postured that it's now quite a lot more likely in the life of a something as big as a 1T drive and this helped prompt my decision to now use a ReadyNAS Duo (in RAID 1) as my backup device for key data on my Qnap. I think it's a great shame that the NAS manufacturers don't explain this (and TLER) to folks along side the hardware compatibility list table as it's very important. I believe that folks should chose these enterprise (RAID class) disks unless they are operating on a shoe-string budget. For SOHO and above business users, is't a 'no brainer' and it concerns me that the less technical business user will likely buy desktop disks; there is simply nothing to say why they shouldn't and that really should be addressed pretty swiftly!
Incidentally, that same friend had a RAID 5 disk failure then slotted a new one in. The stress of rebuilding the set prompted the 'death' or read errors from another old disk (can't remember which) and that was the end of the RAID set! I have heard of others having the exact same problem and for that reason alone, I'm now using my Qnap with 4 disks set to RAID 6 (with the correct enterprise disks, of course) and my RAID 1 backup (again with the correct enterprise disks) will hopefully not ever be required unless I get a major catastrophe with the RAID 6 Qnap (like the shelf falling off the wall and breaking all the disks, or, of course, fire/theft).
The off-site USB backup is my third line of defense; it is stored in a fire-proof box in my parent's cellar.
My end plan is to locate the ReadyNAS in my parents house and rsync the Qnap to it over the www.
Paranoid; me; naw, surely not!