If you connect a device to your computer that already syncs with a different Windows Media Player library, you can choose to sync to the current library temporarily, or you can choose to switch the device to sync to the current library permanently.
If you choose to sync temporarily only, certain Player features are not available. For example, any changes to the media information on the device will not be copied to the file in the library. Also, the library that has the permanent sync partnership with the device will not be able to remove any files added in the current session, and you must manually remove the files.
If you decide to sync to the current library permanently, any existing files on the device will remain on the device until you remove them manually. To remove files manually, do the following:
Start the Player, and then connect the device to the computer.
If necessary, click the Sync tab, and then, in the Navigation pane, click the device.
Right-click the file or files that you want to remove from the device, and then click Delete.