With filemaker server 10, you can't back up to an external drive such as another computer. I suspect the same is true for filemaker server 9 as well.
To back up to an external location, you have to back up to a local directory and then use a system script such as VB Script, a Batch file or a mac compatible equivalent to copy the files accross the network to the other location.
welcome to the only software I've ever come across to not access network paths.
Look into scripting something up using rsync.
Ths your reply