Whether you use Time Machine or not, you will probably have been irritated by the constant questioning of whether you wish to use an external drive to backup with Time Machine to it.
If this is the case then there is a simple way of disabling this via the command line
Open Terminal & type
sudo defaults write /Library/Preferences/com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool YES
And hit Enter, you will no longer be prompted by Time Machine, if for some reason you miss the prompts then just swap the YES for a NO and you will be presented with them again
Once disabled, you can still choose which disk to use for Time Machine by using the options in System Preferences.
Open System Preferences
click on Time Machine
Click on Select Disk and then choose the disk you wish to use