1. Open the Sharing preference pane and check that the computer name hasn't changed without your knowledge. For example, if the name was "X's iMac," it might now be "X's iMac (2)". In that case, restore the original name. You may have to click the lock icon in the lower left corner of the preference pane to unlock the settings. Test. If there's no improvement, or if the name had not been changed, continue.
2. If you're backing up to a Time Capsule, or an AirDisk attached to a 2013 or later model AirPort Extreme base station, restart it in either of the following ways:
☞ Disconnect and reconnect the power adapter.
☞ Select the device in the AirPort Utility application and then select
Base Station ▹ Restart
from the menu bar.
If AirPort Utility notifies you of a firmware update, install it.