Don't offer new disks for Time Machine backup 
Prevent Time Machine from prompting to use newly connected storage as backup volumes.
- Tested on macOS: 
- Catalina
 
 - Parameter type: bool
 
Set to false (default value) 
When a new disk is connected, system prompts to ask if you want to use it as a backup volume.
bash
defaults write com.apple.TimeMachine "DoNotOfferNewDisksForBackup" -bool "false"Set to true 
When a new disk is connected, system does not prompt to ask if you want to use it as a backup volume.
bash
defaults write com.apple.TimeMachine "DoNotOfferNewDisksForBackup" -bool "true"Read current value 
bash
defaults read com.apple.TimeMachine "DoNotOfferNewDisksForBackup"Reset to default value 
bash
defaults delete com.apple.TimeMachine "DoNotOfferNewDisksForBackup"