Hmm...
If they are supposed to be exact like for like copies then you could use a batch file.
Something like...
That will go through all of the files in you originating drive (C:) and look for the same file in the destination drive (Z:) and if it can't find the latter it will put the file name of the former into a text file called "Files not copied.txt" on your C drive.
I'm sure there are probably better ways with software etc... but that's the quickest way I can think of off the top of my head.
You can change the C:\ to be C:\music\ and it will search all files in that folder and subfolders.
Incidentally, the long files names are caused by having files in lots of subfolders and then renaming the higher up folders.
Only way round it is to find the files and then rename one of the parent directories until the entire file path is shorter than 255(?) characters.