Re: Is SSD really the reason?
HDDs are the most reliable capacity medium at the moment. And honestly there is no reason to run a multi-terabyte NAS on SSD for the average user.
That said, if you are using your NAS/SAN for both storing large files and doing a lot of random IO on small files, you should look at ZFS with an L2ARC. I use the FreeNAS distro for this, and the ssd acceleration really helps.
> with five 4TB drives (16TB net)
Sounds dangerous. Raid5 style systems shouldn't be implemented on larger disks due to the much larger chance of multi-disk read errors on full rebuilds. (and BeyondRaid is a type of raid5 unless you have the Pro unit)