Thank you all for the suggestions
Drivers are all up to date - Disk manager won't allow drive letter rename on the problem HDD. I renamed my PC's "F" and reattached the external - still not recognised. Under disk manager it doesn't show a file system but properties shows it as being FAT32. There is no Volume name available and continues to confound me. It's possible that it may have been attached to a MAC previously but with the file system being FAT32...?
I'll attach it to a MAC and see what happens.
Again - thank you all. Enlightening and appreciated.
SF