Was the computer finished copying the first group of ten before you tried copying the second (or third or fourth) group? If the computer wasn't finished copying you might have created a "buffer overflow" problem. A buffer overflow is when a computer tries to store more data (your music files) in the temporary data storage area (the computer memory) than it's able to hold. Everything just hangs while the computer tries to figure out what to do.
What I meant by "expanded disk management window" is when you first open Disk Management the screen might be minimized. You need to make sure you expand (maximize) the screen so all information can be seen.