If you have a large storage iPhone, you might not have the space to back it up locally on your hard drive, especially if you’re working with a laptop’s SSD. OS X Daily points out that with a little Terminal magic, you can set up iTunes on your Mac to back up your iPhone to an external hard drive.
While a lot of us just back our phones up to iCloud, that’s not exactly the best option, especially if you want control of those backups. So, locally is the way to go. For local backup, you use iTunes, which somewhat obnoxiously decides the folder it’ll use for the back up without your input. Thankfully, you can get around this by using a symbolic link in OS X:
On your external hard drive, create a new folder where you want the backup to go. Name it something you’ll remember, like “iPhoneBackup.”
Open a new Finder window, then tap Command+Shift+G. Enter this in the box that pops up ~/Library/Application Support/MobileSync/ then find the folder named “Backup.”
Copy the “Backup” folder to the folder you made in Step 1. Then, rename the original Backup folder “Backup-Old.” If you need to conserve space, you can delete the “Backup-Old” folder once you verify your backups are working in iTunes.
Now, launch Terminal and type in the following command, replacing the “ExternalHardDriveName” with the info for your external hard drive: ln -s /Volumes/ExternalHardDriveName/iPhoneBackup/Backup/ ~/Library/Application\ Support/MobileSync
Go ahead and open up iTunes, connect your iOS device, and back up your iOS device as you normally would. The backup file should now be on your external hard drive.
From here on out, iTunes will always back up and restore your iOS device from that external hard drive, so make sure it’s connected to your computer when you’re doing a back up. Head over to OS X Daily for a bit more detail about the backup process as well as another method for doing so.