Software to allow saves to external drive when it's not present? I'm wondering if software for this purpose already exists.

What I want to be able to do is to be able to save to my external drive when it is not connected to my computer. By this I mean I want the drive to always appear as though it is connected so that I can choose to write to it whenever I want. If the drive is connected when I save to it the files I want to save will be written to my local drive and then when the external is connected, copied across. 

Does anything similar to this exist?
Maybe not in the form you want, but have you thought about using rsync via Terminal?

i.e., rsync –r /Users/you/Documents/ /your_network_mount/destination_folder/

Then either run it from the Terminal whenever you need to, or save the above as a .command, create an Automator workflow, and run the .command using "Do Shell Script". If you save the workflow as an Application, you can just double-click it and voilà.. instant directory syncing to your external drive at your fingertips.