Apple’s iTunes store offers a mixture of protected and unprotected music files that have restrictions on usage, which can lead to complications for iPod users and people trying to share music. iPod users occasionally get a message that a song they have is not authorised to play because there are no more authorisations left for the song. This problem can be fixed on your computer using iTunes.
Open iTunes on your computer and select “Store > View My Account” from the application menu. Enter your Apple ID and password in the dialogue box that appears.
Click the “Deauthorize All” button in the “Apple Account Information” window. This resets the set of computers that are allowed to play music from your iTunes account. The computer you are using will automatically be authorised as soon as you attempt to play or sync a song from your iTunes account on it.
Connect any amount of iPods you like to your computer (one at a time) and add the previously blocked music by dragging it to the iPod icon in the “Devices” column at the left of the iTunes window. The music will now play in any iPod you sync with without any problems or messages.
iTunes music can only be shared among five computers at one time, but it can be shared with an unlimited amount of iPods or iPhones from any single authorised computer, thus, there is no way or need to add or delete an iPod to or from an iTunes account.