Re: RAID6
Rebuild times for classical RAID (including smarter ones like ZFS) is a bit problem with modern drives because the capacity has increased way beyond the read/write speed, so you can be looking at days or even a week or so. That is not, in its self, a problem but both the longer time and the huge amount of data means you have a much greater chance of another disk croaking (or discovering bad sectors) during this process.
This is why you really, REALLY, should be scrubbing your RAID array every week/fortnight. This forces the disks to read every sector and then to fix/remap bad sectors while you still have parity, so when you lose a disk in RAID-1/5/10 you have a sporting chance of a successful rebuild.
Better still, look to dual parity like RAID-6 or ZFS' RAID-Z2