1 – Plug an external hard drive into your Mac’s USB, FireWire, or Thunderbolt port (depending on the type of interface the drive supports).
2 – If a window pops up asking if you want to use the external drive to back up with Time Machine, click Use as Backup Disk.
Note: You’ll have the option to encrypt the contents of the backup if you want. Just be sure not to lose the password if you choose to use encryption!
3 – If you aren’t asked if you want to use the external drive to back up with Time Machine, click the Apple icon on the left-hand side of the top menu bar.
4 – Click System Preferences>Time Machine.
5 – Click Select Backup Disk, then choose a backup disk from the list and click Use Disk.