Build your own and use one of FreeNAS, Rockstor, or roll your own (as I have with ZFS on Linux). I can't recommend any competitors specifically since I've just

Build your own and use one of FreeNAS, Rockstor, or roll your own (as I have with ZFS on Linux). I can't recommend any competitors specifically since I've just been building my own, but last I looked drobo didn't look that bad.