Don't use RAID5 with drives over 1TB.
a) a RAID5 rebuild takes many hours, b/c it involves reading the entire disc.
b) drives from the same production batch tend to cluster failures.
c) I recall reading that the uncorrectable read error rate tends towards the 2TB mark.
That is, chances are very good that a single drive failure will become a 2-drive failure during a rebuild.
RAID6 or nothing.