That’s weird.
But perhaps it´s not the HDD after all. The more I think of it the less I´m certain about anything. 
But thanks for your additional comments. Very interesting.
P.S.:
I also have issues regarding network.
For that I posted a separate thread here: Cannot get "barrier" to work .
Anyway thanks a lot for your help. It´s very much appreciated.
Greetings.
Rosika 