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Time Machine troubleshooting
The backup disk is unavailable or can’t be found or “mounted”
See:
If Time Machine can’t find your backup disk
Use a password to unlock your backup disk
Apple Support article: Time Machine - Backup volume could not be mounted
An incomplete, interrupted, or failed backup occurred
See:
If a Time Machine backup is interrupted
General troubleshooting
Make sure your backup disk is plugged in, turned on, and securely connected to your Mac. If your backup disk is on a network, make sure both the disk and your Mac are connected to the network. If the network is experiencing problems, your backup disk may not be available.
Download and install all recent updates to macOS. If you have a Time Capsule, update it with the latest software and firmware. For more information, see Keep your Mac up to date and the Apple Support article About AirPort base station firmware updates.