It's either the fact that your disk image can't be written (set as read only), but that's obvious enough you thought about that.
it's a permission issue : I experienced it with a ram disk, it was a bit of pain to solve it, but it was definitely a permission issue.
I checked (one can never be too careful) the disk image write status (using Disk Utility), it is Read-Write. The option to repair permissions does not appear for the mounted disk image, so no luck there. (I did repair permissions on the hard disk, but that didn't fix the problem)
To check whether the problem might come from user permission problems, I placed a a PDF file onto the same disk image , and modified it (using Preview's Tools - Annotate option). That worked fine, so Preview.app and the Mac OS X userid I am using can write to the mounted image. Does this mean that Filemaker.app itself somehow lacks permission to write to mounted disk images ?
you have to check the permission on each FP7 file you have with the bottom of info window (command i). You can compare the settings to those of the pdf file
Repairing permission does noting on non mac os x installed and some application files
You could also trye to create a new fp7 file on the disc image from FMP directly and see if it's working
Permission of the FP7 file and of the PDF file are identical.
I've changed the permissions on the mounted disk image to give "everyone" R/W access (previously only "my" user had R/W, "everyone" had "no access"), and to apply these permissions to enclosed items. No joy!
I've also tried to create a brandnew database directly onto the disk image. This doesn't work either.
As always I am able to modify a PDF file stored on the disk image using Preview.