GDIVA is a tool to easily upload files to a DIVA mp3 player or similar.
The problem I found using "cp" in command line is that you don't know the time it's gonna take, and with Nautilus you can't copy the files in the order you want.
So this program lets you easily compose a list of files to copy, even files taken from a playlist, reorder the list by drag'n'drop, until it reaches the limit of the device capacity, and then, by pressing start, copying all the files, showing complete information of remaining time, and progress bars.