How to Back Up iPhone to an External Drive Windows 10

Here's how I solved the problem on my Windows 10 machine.
First, since I had the capacity on my current drive (but only just),
I created an iPhone backup using the regular iTunes sync method to the default location.
I went to the folder Apple uses to store backups, typically this one:
C:\Users\[Username]\AppData\Roaming\Apple Computer\MobileSync\Backup
You should replace [Username] with your own actual username, naturally.
You'll want to copy that folder to your desired new backup location,
and then either delete the original Backup folder or rename it as "BackupOld."
Then hold the shift key and click the right mouse button to open a command window.
There, enter the following:
mklink /J "%APPDATA%\Apple Computer\MobileSync\Backup" "[External Drive]:\iTunes Backup"
Of course, you'll replace [External Drive] with your actual drive letter.
You can add subfolders here too if you like to keep your storage as organized as possible. So, something like this would work just fine:
E:\MyBackups\iTunes Backup
Then you can close the command prompt window, and try an iPhone backup to see if it works.