I guess this is a recent restriction and the creation of static volumes won’t work anymore with QTS 4.3.4 or above, as written in the current release notes (see post above).
Christopher, first of all let me say that I greatly appreciate your work on getting Roon to the QNAP platform!
I find that line in the release notes rather ambiguous: it says it is no longer possible except when mounted in internal slots or on QM2 cards, is it possible to mount them anywhere else?
So I tried it on my own NAS (on QTS 4.3.4). I removed it from the cache position and created a new Pool and a test Volume. This works just fine, see attached screenshots.
I wonder if I would notice any performance boost when I would move the Roon database to this volume with 1.49GB/s transfer rates according to the performance test! Didn’t test this and just switched the M2 strip back to its original role: boosting the overall NAS performance by working as a cache.