are you running windows 7 on the computer? i had an issue where it would start to copy and then stall, after searching through the forums (don't remember which one, sorry) i found a post suggesting the following which works reliably for me:
do not open the folder you are trying to copy *to* in the display pane (right side) of windows file manager. instead, keep the file manager display one level higher in the directory tree and drag/drop the files you want directly onto the folder icon. i know that's not very clear... the theory is that windows has a hard time "keeping up" if the folder you are copying to is open in the right pane while copying. works every time for me. i'll hunt around and see if i can find that thread because i'm sure it makes more sense than what i just wrote
EDIT:
found it here, post #27