I have been trying to change the directory where backups are saved from filemac:/Server HD/Library/FileMaker Server/Data/Backups/ to filemac:/Data/Main Data/Dropbox/Backups/ or any other directory on that drive or any drive on the mac mini server. and get the same error message "Not a valid path" I have tried on the same drive even added
filemac:/Volumes/Data/Main Data/Dropbox/Backups/ but nothing but that the default path will validate. I am frustrated at best. I am running updated Mac OS 10.8.3 WIth Server. This works fine on a Windows 2008 Server. You would think since Filemaker is owned by Apple it would work better on a Mac Mini Server. but it was a problem to install.
You have to set the permissions correctly for this to work on OSX. The owner of the folder needs to be fmserver, of group fmsadmin