I’ve settled on the OWC Thunderbay 4-bay Thunderbolt enclosure as the go-to solution for storage on my 2013 Mac Pro (three of them now attached). It’s such a clean solution with 4 bays via Thunderbolt—what a relief from the days of eSATA and MiniSAS.
When the 3rd unit arrived, I inserted two SSDs into it, turned it on, and was disappointed to find it wasn’t working. Well, what I mean is that the fan was so quiet I thought it wasn’t working! Putting my ear close to the unit, I could hear it running and indeed the drives mounted and all was well. With hard drives installed, “spinner” noise will dominate any fan noise.
If you have one of the original builds (much louder fan), get a replacement quiet fan for the OWC Thunderbay (for units produced early in 2014—builds as of March 10 had a quieter fan and April builds might have been still quieter).