Mac question

I assume its a USB drive. The first thing I would do is try a different USB port. There may be a problem with the one you are using. After that, try rebooting your computer with the drive already in place. If it still doesn't work, ask your friend what file system he formatted the drive with and what format is the music in. It may not be compatible with Mac OS.

There are some other things you can try, but I would do this first.

I understand the need to go through iTunes to access the music (if he's using iTunes as his player), but you should still be able to see the files themselves in finder. That's why I think the issue may lie elsewhere.