UPDATE
So two more of my drivers start to behave just like my DATA drive. Again only with this OS (with UAC)
The other system as always has no access issues to any of, now in total; three drives...
During this problem with the original DATA drive, I was using this external drive just fine like every other day...
Prior to this external one, there is no event that could be a cause; like a failed Restore Point or some other error containing a major event.
(And there were another 1TB HDD which was also only readable. But I reformated it for restoring Images anyway...)
Unless this thing is contagious through drives, this is bound to be a Windows dance, no?