I don't think MC likes trying to delete the active library
You cannot
delete the currently active Library. If the UI lets you, it shouldn't, because it isn't reliable. MC
does not run without a Library loaded, so deleting the current library out "from under it" while it is running is a Very Bad Idea. You can Clear it, but not delete it. To delete a Library, you
must load another one first.
And, you
also can't delete the original default Library at all (that's always reserved as a "fallback location"). Not without some trickery, at least, which is best left unexplored.
Renaming Libraries should work, but I'd probably make it non-active first, even if that isn't absolutely required.
In any case, if all else fails, simply:
1. make a backup
2. add a new Library to MC where you want it on disk and with the proper name
3. restore your backup (Library Only) into this new one
4. then delete the old one