Further testing is showing the SATA connections to be rather unstable, even in ROOBI.
Sometimes the OS boots with only the 4 SATAs from the board available, other times only the 6 from the adapter, other times none… and very very occasionally, all 10 are available. It’s a rather tiring game of SATA bingo every time I reboot.
This seems related to Fixing the problem with PCIe/SATA on Rock 5 ITX and will give the fix a go as soon as I have some time.