Instructions
Plug your USB hard drive into an available USB port on your computer.
Open Window Media Player 11. Choose the playlist you wish to copy from the "Playlist" menu on the left-hand side of your screen.
Click on the "File" menu button located in the task bar, then choose "Save As."
Change the destination in the drop-down box to your USB hard drive device, then click "Save." This will cause the playlist to save on your USB hard drive device.
Unplug your USB hard drive from your computer and plug it into an available USB slot on your Xbox 360.
Select the "My Music" submenu located in the "My Xbox" section of the Xbox "Dashboard."
Choose the USB hard drive device from the list.
Choose to filter music by "Playlist" and select the desired playlist you would like to play. This will cause the Xbox 360 to play the playlist from the USB drive.