Optimal RAID 6+0 Setup for 40+ 4TB Disks

With 4 TB 7.2k drives, I'd recommend making the subarrays as small as possible - actually, 5 drives don't really justify using RAID 6 at all.

My 2c are to use RAID 10 where you can expect a rebuild to finish within 12 hours which a 5-drive 20-TB RAID 6 array most probably won't.

Make sure you enable monthly data scrubbing/media patrol/whatever-it's-called-here to detect read errors before they have a chance to stop a rebuild. Most often when a rebuild fails, the cause is not a completely failing drive but a rather old, yet undetected read error that could have been repaired with a regular scrubbing.