Moving my iOS backups to an external drive…

First copy them over by going here on windows:

To Find backups in Windows 7, 8, or 10

1. To find a list of your backups, go to \Users\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\
2. Or you can use these steps:
3. Find the Search bar:
   • In Windows 7, click Start.
     In Windows 8, click the magnifying glass in the upper-right corner.
     In Windows 10, click the Search bar next to the Start button.
4. In the Search bar, enter %appdata% or %USERPROFILE% (if you downloaded iTunes from the Microsoft Store).
   • Press Return.
5. Double-click these folders: Apple Computer > MobileSync > Backup.

source: https://support.apple.com/en-us/HT204215

Now to make it backup on the external drive…

1. Note the drive you want to move it to, for me that is ‘E’
   1. **update D for my windows 10 machine
2. Connect phone to PC, when itunes starts, close it
3. Open a command window and type:
   • mklink /J “C:\Users\JxA\AppData\Roaming\Apple Computer\MobileSync\Backup” “e:\BackupLink\iPhoneBackup”
   • **Update for windows 10:
     • mklink /J “C:\Users\JxA\Apple\MobileSync\Backup” “d:\BackupLink\iPhoneBackup10”
   • NOTE: Ensure that you use ‘Straight quotes (") instead of smart quotes!

source: http://smallbusiness.chron.com/backup-iphone-external-drive-72349.html