If you plug in an external drive that you want to use for Time Machine backups on macOS, and your Mac doesn’t automatically present you with the option to “Set Up,” then:
- Make sure the drive is on
- Click on Spotlight, the little magnifying glass at the top right of your computer screen
- Type “disk”
- Double-click on Disk Utility
- In the left sidebar of Disk Utility, you should see a line with the generic volume name and thumbnail of your external drive, e.g. “LaCie” or “WD” probably with a yellow icon. Click that line
- Click the Erase button on the right
- Set the Name to “Backup” (if you like, put your name in there too)
- Set the Format drop-down menu to “APFS” if it’s not already
- Click the Erase button
- You’ll be asked if you’re sure. Yes, you are sure.
Now, if you haven’t previously chosen a Time Machine backup, your Mac may ask if you want to use the new volume for backup. Say yes. If it doesn’t:
- Go to Apple menu > System Preferences > Time Machine
- Click Select Disk
- If there’s a lock at the bottom left of the window, unlock it with your password
- Click the Options button at bottom right. If there are any items listed under “Exclude these items from backup,” remove all of them except for the grayed-out “Backup” drive
- Click Save
- Click on “Backup” (or whatever you called your drive)
- Click Use Disk.
Time Machine will start backing up your computer. It will give you a broad idea of how long it will take. Whether it’s a couple of hours or overnight, just leave it until it’s done. From then on, Time Machine will continue to back up the entire computer every hour.