I don't have enough disk space to back up my iPhone to my Mac. How can I backup the iPhone to an external drive?

Yes. You can but you need to be well versed with the Terminal app. If you mess-up you can never back up again even on your Mac.

You will have to create a symboling link of the backup. See the Step 3 process on --> https://www.imore.com/how-move-your-iphone-or-ipad-backups-external-hard-drive

Alternatively...

For iPhone backup using iTunes (Transfer to external Hard Disk)

1. 1. Open Finder > Go > Go to Folder... > [~/Library/Application Support/MobileSync/]
   2. Open Backup Folder
   3. Copy Paste the BackUp file to any External Hard Disk

Have you considered a really big external disc? Apple offers this solution.

There is no direct method to backup your iPhone to an external hard drive location. There is a manual method whereby you backup your iPhone on your computer. Then move the backup file to the external drive until needed. If you need to restore from that file then you will have to copy the file back to the original location on your computer first. Yes this is a workaround if you absolutely must store backups on an external drive.

For very detailed instructions on how to do this please click here ➔ How to Backup an iPhone to an external drive.

Axel F.