Permanently disable time machine prompting for external disks
From Turn Off Requests To Use New Disks for Time Machine [OS X Tips]...

You can stop this request dialog box from appearing by opening a Terminal window (Finder -> Applications -> Utilities – > Terminal) and typing the following:

defaults write com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool TRUE


Then log out and back in again for the changes to take effect.

If you’d like to restore the request dialog at a later date, again open Terminal then type the following (log out and back in again after for the changes to take effect):

defaults delete com.apple.TimeMachine DoNotOfferNewDisksForBackup