True, FM can not backup to a network drive.
But there are ways around this limitation.
I created a fairly simple VB Script and used Windows Task Scheduler to copy my latest back ups to an external drive. See the following link for how I did it. It's fairly rudimentary, but a version of this can be made to work for you if you study up on VB Script (or use a batch file the same way) and learn how your systems domains are set up.