There is a setting in Time Machine that causes it to automatically start and do a backup when a new drive, even a thumb drive, is connected. You can turn that feature off in Terminal with the following command:
defaults write com.apple.TimeMachine StartOnMount -bool false
There are a handful of useful terminal commands and hidden marker files associated with keeping time machine on a leash. I'd never heard of it "pouncing" on a drive though without a prompt? ("Use xxx as time machine backup?")
The "never present" key is useful to keep a drive you move around a lot from producing TM prompts on new machines if you move it around like I did with my service drive.
