Let me repeat: "Hard disks are not supposed to sit on a shelf unplugged for extended periods."

This even more the case with the very large capacity disks you are mentioning here.

As mentioned elsewhere in the thread, what about burning them to 50 triple or quadruple layer Blu-ray Discs (BD-R's) then store them in a fire proof safe?

http://en.wikipedia.org/wiki/Blu-ray_Disc_recordable

Every month go fetch the old hard drives, copy all the new data onto them, then take the currently in use hard drives and put those back into storage.

If you want more redundancy replace 2x 2x3TB drives with 2x NASes each with 3 drives in raid 5 or 4 drives in raid 1.

If you archive disks like this, you need to run software like SpinRite on them ever so often to maintain them. I'd suggest that you do it every time you update your archive on the disks.

[citation needed]

A lot depends on whether you're doing a backup or an archive; if it's a backup you'll be rotating the disks in periodically.