I have an R510 as my NAS with an assortment of 2-3TB drives that are pooled with Drivepool and protected with SnapRAID. It's only about half full (8TB) but that means my 8TB USB backup drive is just about full when I mirror the NAS. I'm considering an external 4-bay USB enclosure to attach to the R510 (will need a USB3.0 card) to use as a backup destination. I'd simply pool some inexpensive drives together to expand - no need for RAID overhead since this will really only be a last-resort backup.
That said, USB enclosures don't seem to be too popular here.
That said, USB enclosures don't seem to be too popular here.