MTBF do not add. They are also statistical numbers based on
approximation under abnormal conditions more than real life
experience under normal operating conditions.
RAID-5 does not prevent drive failure - it helps to preserve data
in case of a failure.
Relying completely on RAID-5 to save your arse is foolish, as is
relying on a single backup.
Everything fails/dies sooner or later...usually sooner. And if
your job depends on it, it will fail the moment you schedule
some much needed vacation...
-- Andrew E. Mileski