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How to move your iPhone or iPad backups to an external hard drive
If you have iOS backups taking space on your internal drive, you can relocate them to an external hard drive to clear out some space.
If you do now, or have ever, synced your iPhone, iPad, or other iOS device to iTunes by connecting it to your Mac, you have backup folders stored on your internal drive. It's just the way life is. Maybe you only have a few files from a couple of iTunes backups and they don't take up much space. Maybe you've been backing up your iPhone via iTunes for years and have a space hog on your hands.
If you backup your iPhone or iPad via iTunes, and feel the need to get these iOS backups off of your Mac and onto an external hard drive, it is possible to do, but beware, Apple really doesn't recommend it.
Note: This guide is for people that back up their iPhone or iPad using iTunes. If you use iCloud to back up your device, you can simply delete any old iTunes backups on your Mac. Just make sure you really are backing up to iCloud before you delete anything.