When i go onto my iTunes to play a song i have bought it asks for my username and password. Once this is entered it tells me " you have authorized a total of 4 machines out of your available 5" when i have only 3 machines. I click ok and then get an error message "There was an error storing your authorization information on this computer. The required directory was not found or has a permissions error. Correct the permissions problem and try again, or de-authorize this computer if the permissions cannot be changed." I have repaired permissions using "Disk Utility" and i have de-authorized and tried again. Neither have worked. Any ideas? P.S it is only one out of my three computers this is happening on, the other two are fine.