Backing iOS up to a secondary drive on macOS

So my wife got a new iPhone yesterday which of course means me playing tech support and me backing the old one up and restoring onto the new one. My wife has a Mac Air with a somewhat limited internal drive to there wasn’t enough space to back up the old iPhone (128gb). Easy I thought! I’ll just move the iTunes library folder to an external USB drive and the backup will go there. Wrong! The backups are not kept in the iTunes library so that didn’t work.

After a little research it turned out that iOS backups are kept in ~/Library/Application Support/MobileSync/Backup with each backup being a folder in that directory. Hmmm… Maybe a synlink would work? Oh yes! So here’s what I did:

1. Delete older backups from iTunes using Preferences/Devices
2. Quit iTunes
3. Break open a terminal
4. Move to the folder containing the Backup folder:

   cd ~/Library/Application\ Support/MobileSync

5. Remove the folder (now empty):

   rm -rf Backup

6. Create a symbolic link (synlink) to the directory on the external drive to hold the backup (here the USB drive is called MM):

   ln -s /Volumes/MM/iOSbackups/ ./Backup

7. Open iTunes and backup the iPhone and restore onto the new iPhone
8. Quit iTunes
9. Reverse the process (after having ejected the USB drive):

   cd ~/Library/Application\ Support/MobileSync
   rm Backup
   mkdir Backup

10. Done!