How do I restore an iPhone from a backup located on an external drive?

I was unable to restore from this backup because I had symlinked the iPhone backup's individual folder,

Volumes/External Drive/iPhone Backups/UF34...

into my Backups folder located at

~/Library/Application Support/MobileSync/Backups/.

This causes some issues. What you need to do is to symlink the backup folder into the MobileSync directory in your user folder. In other words, you need to create a symlink for the folder which contains all your iPhone backups (the folders with the weird numbers),

Volumes/External Drive/iPhone Backups/Backups

inside this folder on your disk

~/Library/Application Support/MobileSync/

I believe you need to rename (or delete) the Backup directory which is already on your hard disk if you want to do this.

I realize that this process might be slightly confusing if one has not used symlinks very often, so feel free to leave a comment here and I will try to make this answer clearer if I can.

I've definitely seen issues with the symlink during the restore process in the past, but usually it happens mid-restore rather than not being allowed to restore at all. However I haven't checked much on this since the latest iTunes update.

If you navigate through the symlink in Finder:

• Open Finder, and select the "Go" menu at the top of the screen. Hold the Option key and select the "Library" menu option from the Go menu. (On OSX Lion and above, Library will only appear with the Option key held. On some Sierra configurations, Shift + Option may be needed to get the Library menu item to appear.)

• From Library, navigate to Application Support > MobileSync > Backup.

Are you able to reach your external drive and see the backup(s)?

If so, can you close iTunes and reopen it now and see if they backup(s) show up in Preferences > Devices?