The default music location would be C:\Users\[username]\Music so I can't speak to why you have a music directory on your desktop. The character limit applies to both the source and destination locations. Simply renaming parts of the path to shorten them should resolve the issue.
If the copy source is "D:\My Music\Some folder1\somefolder 2\...\Artist\Album 1\Song 1" name you need to shorten each directory or move the music up to a so the full path is shorter like D:\My Music\Artist\Album 1\Song 1