methanoid

Thanks, worth keeping for future BUT it seems to have been some errant BIOS setting that looks like my CPU was overclocked and not stable...  I put to defaults and its working fine and other unexplained issues seem to have gone. 
 
Once I have encoded another 12k videos I will be wanting to explore power saving anyway :-)

So, New Config and preserve all disk slots and cache slots and everything seems to be working....  (Actually it didnt but once I swapped the Cache and VM drives over it seemed to kick it into realising that nvme1n1p1 was NOT in system and was now called nvme2n2p1.
 
But I am worried... is the NVME dying (nothing in SMART reports indicates), is BTRFS stable or even suitable for cache drive, Would it make sense to buy a 2nd identical SSD and mirror them?