The idea with Time machine is to use it to back up to an external drive. Once connected you would tell Time Machine to use that drive.
However before you do that, connect the external drive and open Disk Utility
1. Click on the external drive
2. Click on the Erase tab and select Mac OS extended, Journaled.
3. Click Apply
When finished it will be formatted correctly and then open Time machine and tell it to use it as the Time machine backup disk.
a handcrafted note by roam