Connect back up disk and it only prepares to back up. Can't get it to back up.

You did not specifically say it refused to backup. If it refused, can you post the error message?

If the Backup Drive has been disconnected for most of a day or more, it may take HOURS to prepare. Once too much time has passed, it needs to do a full traversal.

If this is the way you intend to operate it, you will need to be more patient.

Pondini's Time Machine - Troubleshooting -- D1. Stuck in "Preparing" or "Calculating changes"

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Goes from looking for backup disc to unable to backup because disk (which is the airport time capsule) cannot be found

see if this article is any help:

.Pondini's Time Machine - Troubleshooting: C19. Backup delayed or The backup disk is not available

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