I do the same RoboCopy is the way to go:
Robocopy <Source folder <Destination folder> /e /s /r:1 /w:1 /purge
So to copy all the songs (or just update) in C:\mymusic to the USB drive on "G\MyMusic" it would be:
Robocopy c:\mymusic g:\mymusic /e /s /r:10 /w:1 /purge
/e = Include empty folders.
/s = Include all sub folders
/r = Only do 10 retrys (Default 1 million!!)
/w = Only wait 1 second between retries
/Purge.. Careful with this one.. Using Purge will delete any files on the USB drive that do not exist on the C drive. So the two folders will be a mirror image in the end. If you do not use Purge then anything you delete from the source folder will still exist on the USB drive. Think about that for a sec before using.
You can then make a CMD file that includes multiple robocopy lines for the multiple drives:
Robocopy c:\mymusic g:\mymusic /e /s /r:1 /w:1 /purge
Robocopy c:\mymusic h:\mymusic /e /s /r:1 /w:1 /purge
Robocopy c:\mymusic I:\mymusic /e /s /r:1 /w:1 /purge
Robocopy c:\mymusic j:\mymusic /e /s /r:1 /w:1 /purge
Robocopy has other features such as logging, but for now that's enough out of me