It may also be that you have a bad file in there someplace that stops the process.
I've had this happen often. As soon as Windows hits something that it can't copy it just stops or give you a generic can't do that.
The problem is with as many files as you have I don't know how you would identify it.
What I would try is to make a folder on your external drive called Moving or something like that and then drag say 6 of your music folders into that. This is almost instantaneous since it doesn't really move them, just changes the address.
Then try and copy that folder to your hard drive.
If it works then move the folders you copied back to the original location (if you want them to stay on the external drive) and drag in 6 more and so...