Hi guys!
To get things clear: I used only one drive for WINDOWS (the original HDD) and one for SOLUS (my new SSD) respectively. Never mentioned a dual booting problem or anything similar, so I'm not sure if I've made myself clear enough in my original post or if the problem is that complicated.
I ran either WINDOWS or SOLUS, never both on the same drive!
Ah well...
Problem solved for now:
When I put back the original WINDOWS HDD to try and run WINDOWS again I checked the BIOS settings and selected "UEFI first" where I was given the choice between Legacy and UEFI boot. Then continued the booting process and everything was OK from then on.
Ha ha ha:
After all the hassle I convinced the owner of the WINDOWS laptop to stick with SOLUS. Worked (until further notice...)!
Thanks for your trying to help!
SOLUSfiddler