Hey there all, ok, an update of the "stability saga" .... the tool I am using is the Intel Tuning tool, doing the Stress tests of CPU, Memory and GPX.
I dropped the Storage Space volume that consisted of the internal 1TB WD Blue drive and external 2TB USB->SATA drive and re-ran all the tests. I was sort of pleased that the NUC continued to crash under various conditions (mostly the Mem Stress Test, but occasionally the CPU test, and occasionally not during any test).
Note that temperatures were reasonable (70 celcius for CPU, 66 for GPX) as max.
Next I removed all the App software just in case, still failing randomly.
Next I removed the upper stick of my 2 sticks of 4GB Crucial RAM - I had installed 2 x 4GB Crucial 51264BF160BJ RAM which is approved by Intel. Ran all the tests and after 12hrs had no failure, reinstated my 2 hard drives (internal and external), the Storagepace volume, and all software (Plex, iTunes, Logitech etc) and still no failure. After 3 days of running CPU/MEM and GPX stress tests (not continuously), and general use, it is yet to fail. Hmmm ... so either a faulty stick of RAM, faulty seating or faulty adapter
What next ... I'm running stress tests all day, if pass, I'll swap the RAM for the other stick and start testing all over again. If that passes ... not sure ... maybe add back the second stick and see what happens.
Thanks, Steve