Right! For anyone else who stupidly, inadvertently and mistakenly moves their library file to a base drive like F: or D: without creating the actual containing directory like "music" or "documents" etc, and on trying to relocate it elicits the error message, "Failed to build the list of regular subdirectories under...<insert drive>. Access is denied", and is, like me, NOT a computer expert, the instructions for OTHER dummies is:
Make a dummy temporary folder called "music" or "documents" or whatever your library is on ANOTHER DRIVE (it won't allow you to move a library to a subdirectory of the drive where it currently is). An external drive is fine. It doesn't matter where, other than that - it's just temporary - and it requires no space, so an almost full drive is OK.
Move the library to the temporary location, BUT when the pop-up asks you if you want to move the files as well, say NO. Then it will just create the empty directory that you have named (music, documents etc) leaving your actual files where they are - the main thing is that you have moved the directory recognised by windows as the library of music or documents or whatever (your actual files are still safely in the wrong place).
Create the new "music" or "documents" or whatever file in the CORRECT location.
Move the windows library directory a second time to the correct location - it's still an empty file.
Move your files manually into the directory.
Delete the temporary/dummy folders - they were just a temporary location and should have remained empty.
Phew!! All good!!
I went all over the net to find this! Lots of questions from OTHER people too on how to fix it and no bloody answers! Someone mentioned the possibility of this as a passing comment, thank goodness! Strewth!!