Dark1, I'm experiencing the same issue. I can't copy my iTunes music files over from my external hard drive without getting an error message once it starts copying. If you figured it out, could you offer your solution? I'm desperate for help here. There are no issues with either drive according to the disk repair drive.
Note: My music was originally stored on a Windows CPU. When it crashed, I was able to move all the contents of my hard drive to the external HD. Now that I've got a new iMac, I'm trying to move all that music back to an internal HD.
Any suggestions?