I had similar problems with a single Drive in a new TrueNAS setup. The Drive would come up healthy after most reboots but after some reboots it was unhealthy. For me S.M.A.R.T data die not indicate and errors. I reboot the Maschine often because it is a backup system that only runs during backups.

Swapping drives (with a known gold drive) did not resolve it. The error was always at the same Drivebay.

For me thepProblem was the y-split SATA power cable I used. After replacing it the system is working without a problem since.