I just did this for my iPhone 6 backup on my PC (Windows 10).
I followed the instructions from this website:
http://smallbusiness.chron.com/backup-iphone-external-drive-72349.html
As a side note, I did run into a problem where it kept saying the file was created in the msdos cmd window when I was trying to create the symbolic link. I ended up deleting the "Backup" drive first in the C: drive, and then typing the mklink cmd again. I found that solution here at this website, while also has other solutions to other similar problems as well (just in case you encounter something similar to me).
https://superuser.com/questions/597551/mklink-error-cannot-create-a-file-when-th at-file-already-exists
Hope that helps. Goodluck!