It appears you have gone down the Rabbit hole with that install. You may never be able to get it to work, and if you do it may be so corrupted your mother will not be happy with it.
So, three thoughts:
You should not need to go through Vista to get to Windows 7, XP should be upgradeable to Windows 7.
If you were to clone the drive to another drive on the same system, it may be forced to go back to a C: for a system drive. I have seen cloning operation change the drive letters because it cannot have two C: partitions (or J).
Changing system drive letters on an installed system may have repercussions such as the registry still having the old letters and also boot failures.
But the best, most logical way to go is to do as suggested and perform a clean install of Windows 10. If your mother needs old files they should be available on the old drive or the image. Trying to drag old programs and drivers into Windows 10 is not a good thing to do.
I couldn't upgrade Windows XP to Windows 7, I tried that already, that's why I did XP > Vista > 7 > 10.
Everything was working correctly, until I added the harddisk. After that it went nuts. It was a blank HDD. Before that the SSD (with Windows 10) was working properly. The only annoying thing (and most probably the problem) was that J: was the system disk (where Windows was located and program files) in a normal system this would be C: obviously. But as I mentioned above, this was caused by restore/backup from years ago and was never resolved.
Clean install is not an option. She still has her old Windows XP PC. And I doubt I will ever redo this again. Her old PC already has bad clusters / sectors, that's why I decided to clone everything onto a new PC along with updated Windows. Which worked, until I placed the harddisk in there (for extra space and storage).
I know drag & drop is not going to work. But her old PC contains old programs with serials, she bought, but don't exist anymore or aren't retrievable. That's why I went through the cloning process.
The only good side; is that her old Windows XP PC has a RAID 1 system. I didn't want to do this before, but is it possible to take out the 2nd drive from this system, replace it with a similar or bigger drive and use the removed disk in the new PC?
That way it would at least save me the cloning progress. Also will the normal disk be mirrored automatically onto the new harddisk in the RAID 1 system?
But still, the problem will be the same in the end. Everything up and running, install the HDD into the new system for extra storage (pics and backups) and it will go haywire again, just like now...