Thank for that advice.
I've checked the hard drive with Seatools - it passed all tests , also the long generic test. I 'cleaned' the drive with Diskpart in Win 7 .
I then re-inserted it in the NAS. (The faulty one was the RIGHT slot).
Now when I select Management SMART test , both drive are listed and both pass the SMART test....
When I select Management -> AutoRebuild config -> and select Manual rebuild, it says (in status bar bottom of screen "Waiting for 192.168.1.102" , then quickly flashes to
' Hard Drive Config' -> Initialising.... '
Then quickly flashes another message under the initialinsing progress bar ( too fast to read ..something like SWC _ done ??) .
And then quickly returns to the Hard Drive Config menu ....
Only the power blue LED and the RIGHT blue LED are lighting now.. No indication from the left LED.
Now I'm puzzled
Is it rebuilding ?
Thanks again.