Hello Rory,
Since you say that they media attention light came on after your last cleaning have you changed your cleaning tape? I know you stated that you checked the magazines in the RMU and didn’t see any media that had any errors. Also if you setup the auto clean & cleaning slot you need to reduce your available slots by 1 from 24 to 23. Also if you have setup the I/O slot then you will need to reduce your available slots by 1 as well. To reduce the slots you can change it in the RMU by going to Configure Library MenuGeneral.
Please let us know if you have any other questions.