By default, when you are using Time Machine in Mac OS X, every time you insert a drive the system will ask you if you would like to use that drive as a Time Machine backup destination. If you are like me and you swap drives around a lot then this can get annoying. So to stop it you can actually just disable a launchd System Daemon, com.apple.backupd-attach. To do so, simply move the /System/Library/LaunchDaemons/com.apple.backupd-attach.plist to another location and viola, on the next restart when you attach a disk Time Machine won’t ask you if you wish to add the disk to your Time Machine destinations.
While this is one way, reader @Chealion pointed out on Twitter that the following command will go ahead and do the same thing without monkeying with launchd:
defaults write /Library/Preferences/com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool true;
