Oh, and I suppose I should mention the other two methods I had thought of to see if those are possible:
1. "Save a copy" of the database to iDisk each time I finish using it. (I have tried doing this but the "Save a Copy" option in the file menu is greyed out and the script command doesn't work either. I assume it has something to do with the privileges I have set myself - is it possible to "turn on" the option of clients saving a copy of the database locally? It's only me who uses the database, so there are no security issues here.)
2. Set Filemaker up to back up to each of my computers each night, an hour apart, say.
FileMaker Server does not allow a scheduled back up to write to an external drive--so this is what's keeping that approach from working for you. We routinely send back up data to a different computer on our network by using a system script to copy the locally backed up files on the server to a shared directory elsewhere on the network. We use windows, so you'll need to find mac equivalents, but I used a VBScript that is controlled via Windows Task Scheduler to start up 30 minutes after the scheduled back up.