Thanks Martin.
Well, it seemed like too much of a coincidence... and possible explanation. PC works fine most of the time, just causes a lot errors on HDD's and sometimes random shutdowns. SMART OK most of the time. Seatools unreliable. Once HDD 100%, then unrecoverable errors... go figure...
Recently both my HDDs started experiencing problems, HDD repairs are not very successful (spotty) indicating possible I/O issues.
Checked HWiNFO32 and on System Summary Screen I can see 2 or often 3 regularly flashing lines (whole lines):
Northbridge Status (all values - whole line), and ...