I had "location not available" too at first. I solved the problem by formatting the new drive first. Make sure it's FAT32. I used an empty USB stick. Make sure everything on there is temporarily moved, otherwise it will get lost during formatting. In Computer, right-click and choose format. After that, create a new folder in it called iTunes and a subfolder within that called Backup.
In the original MobileSync folder on drive C:\, I made sure, that in the command line, the new location is divided by backslashes. Like so
mklink /J "%APPDATA%\Apple Computer\MobileSync\Backup" "F:\iTunes\Backup"
press ENTER. Notice that the slash before J is a forward slash and all others are backslashes. Now try clicking on the new "Backup" in the original MobileSync folder and create a textfile.
It showed up perfectly for me. Of course to continue with this, make sure the stick is plugged in when a backup of iTunes is performed.