There may be issues on recognizing USB devices, including MP3 / MP4 players, by a Mac computer after the OS is upgraded to version 10.7.2 . This can be solved by resetting PRAM (Parameter Random Access Memory) of the computer:

1. Shut down the computer.
2. Locate the following keys on the keyboard: 
   • [Command]
   • [Option]
   • [P]
   • [R]
     
     
     
3. Turn on the computer.
4. Before the gray screen appears, simultaneously press and hold [COMMAND], [OPTION], [P] and [R] buttons until you hear the second startup sound.
5. Release the buttons.