Well all I can say is thank all that is sacred that I decided last week to move all the data off my Mac onto my NAS and to finally upgrade from Lion (2010 OS...!!!) to Sierra.
I have been going back and forwards installing Sierra, the restoring Lion because I forgot to copy off my iPhoto library of over 40,000 photo's, then reinstalling Sierra etc.
Well the computer has been glitching for about 6 months, slowly getting worse with screen artefacts appearing and it needing a reload every day whereas it used to constantly stay on for weeks/months at a time.
The artefacting appeared to have settled down on first install of Sierra but came back with a vengeance after a day or so. I did another clean reinstall, or at least tried to, and fully cleaned and formatted the HD first, but on trying to reinstall Sierra the machine is now telling me the HD has S.M.A.R.T errors and cannot be repaired. I need to replace the HD!!
So glad I finally found that mojo otherwise I could have lost a LOT of stuff!!!
Cheers
Mark