They have three raid-6 volumes per machine, so they probably leave it in the rack until two of the volumes are unusable and then refurbish the machine.

I doubt it very much. When drives start failing out of a RAID6 array, you lose it's ability to automatically detect and correct single bit errors on disk.

With that much data, single bit errors will happen predictably.

If their usage patterns are anything like ours, disk failures are well under normal rates. Most backup data is stored and ignored. The drives just don't get much stress. Downing a machine for maintenance is probably acceptable to their usage pattern.