Backup Disk Not Found
My friend's TimeMachine can't find her back-up drive ("Disk not found"). We turned the computer off and confirmed the connections are good. Didn't help. We used Disk Utility and confirmed the back-up disk was readable. It was. We had Disk Utility do a "First Aid" check on the backup disk, and it checked out good. But TimeMachine just can't see it -- No Disk can be seen by TimeMachine. We checked preferences, and all the obvious stuff. I thought her passport external disk must have died -- but Disk Utility says it is fine.
Any ideas?
My friend has an iMac 21.5", Mid 2011, and is running OS 10.11.6 El Capitan.