G'day, thanks for helping.
Excuse me for the late reply, I've been busy getting another SSD to make backups of the System before
this problems' troubleshooting is attempted.
I've noticed that the program GSmartControl performs a surface test of Hard Drives. My system now runs SSDs.
I swapped out the original HDDs, and now run Win10 from drive C: on a Samsung SSD 850 PRO 512GB.
All of the OS runs from the Samsung drive, except for the 'Documents' and 'Downloads' folders that I've deliberately
moved to the second internal SSD ( partition I: of the PLEXTOR PX-512M5Pro SSD) of this laptop.
There are also some files and folders that windows has put there, and other files and folder (not OS) that I've decided
to place there, as below.
The output of GSmartControl Extended Self test is as below.
It has been run several times because I couldn't believe that I hhad done it correctly,
as each test only took a few seconds to complete.
There was no highlighted entries in the Attributes Tab
Thank you for your time.