If you don’t use Time Machine, you might notice that every time you attach a empty hard disk to your Mac, you get asked if you want to use this hard disk Time Machine backups. Disable this dialog box with the following command:
defaults write com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool true
Restart your Mac or log out and back in again for the changes to take effect.
Revert to defaults setting:
defaults delete com.apple.TimeMachine DoNotOfferNewDisksForBackup