Re: Agreed on all counts
Pascal, I agree with all this but I think RAID5 is a very, very bad idea at these disk sizes. If one of your drives failes, a single Unrecoverable Read Error on one of the other three drives is going to kill your array. If you are using WD 3TB Reds, with a URE probability of 1e-14, and one drive fails, the chance you can rebuild your array is less than evens .
Add to that your rebuild time (days, I should think), you have a significant possibility of a second disk failure (especially if you haven't sourced your disks from different batches).
In my opinion you'd be much better off using RAID10  and getting 6TB. You've lost 33% of your capacity but really increased your data safety. Although of course, RAID <> Backup :-)
 Chance of success, simplifying somewhat, is no better than the chance of reading each bit successfully (1-prob(URE)) raised to the power of the number of bits 8 x number of disks x capacity of disks; i.e. (1-1e14)^(8*3*3e12)= 48.7%
 Although I'd be tempted to use RAIDZ2 rather than a HW RAID10