Backup Preferences should be in the Lightroom Preferences and no it just buried in some exit dialog

[Cletus]

Since I started helping Lightroom Users, there have been too many occasions where the user had difficulty getting the backup location to "stick" or to be changed to an external drive where it logically should be. 
In addition to the exit dialog, there should be an option in Catalog Settings or Preferences to set the default location for backup of the Lightroom Classic catalog. 
If an external drive designated for Lightroom Backup is not present on exit, Lightroom Classic should pop up an error message and wait for the user to correct the problem before continuing or reverting to a backup folder in the Lightroom folder where it is least useful. 

[cmgap]

It's a good suggestion  to have  a message pop-up alerting the user that the destination is not available as is the case upon import. Not sure if this has been suggested before but you can add  it  here: https://feedback.photoshop.com/photoshop_family/categories/photoshop_family_photoshop_lightroom?topi...