There seem to be two possibilities.
1. The AAMUX is bad.
2. Over time the contacts have become corroded and they are not making proper contact.
If the problem is contact issues, power down, take the drive carrier out, remove the drive screws, then remove and reinsert the drive several times to wipe the contacts. If this does not work then the AAMUX may be bad.
I don't know about availability of the AAMUXes, but as a workaround you can use the Ex10 single controller. Make sure that all the drives are visible to, say, controller 2, and if they are power down the Ex10 and unseat controller 1, don't remove it, or the unit won't cool properly, just unplug controller 1 and pull it an inch or two out, then reboot with just controller 2. If all drives are visible you should be up and running. This may require some changes at the client computer side.