Hmmm… I have no experience with this het but was planning to add a Terra master D4-320.
So far i encounter discussions stating that because it has no integrated raid-controller, and as long it is listed as “single disk” with USB UAS active the individual drives are presented to the OS as individual sdbX (with pool/vdev/raid configuration done by OS).
Okay, edit2add: this user experience is truly adding value to this topic: Do the TerraMaster "D" series (enclosures) play nice with TrueNAS? - #6 by StuPo
The TerraMaster DAS is actually working like In would want it, but apparently it is quite exceptional in doing so.