While Apple is busy pushing its iCloud services these days — which you can use to backup data from any Apple device — it’s understandable if you prefer a more traditional method of backing up your data.
Let’s go over how to backup your Mac to an external hard drive.
Re-formatting will erase any old data that you have on the drive, so you’ll want to double-check and move any valuable data over to another device before moving on.
Launch Disk Utility: Disk Utility is located in Applications, under the subheading Utilities.
Find the hard drive you want to reformat: All drives and volumes on your Mac will be listed in Disk Utility, so make sure you pick the right one.
Erase the drive: Choose to Erase in the Disk Utility window, confirm that’s what you want to do, and get ready to rename the drive — it’s a good idea to call it something like, “Mac Backup” if you are using this hard primarily for backup data.