I'm having a similar issue, and so far all the info that I can find FM Server won't allow you to store external data anywhere but on the same drive as the server installation.
In disucssions of the new FMP12 storage options with FMI engineers, it has been noted that served files have limitations regarding external storage, and that external storage on served files should be set to Secure, not Open, for optimal use. Nobody should have access to the served container data via the OS-level in any event.
The option to use Open external storage was an added feature, intended for user convenience on local files, with the caveat that you still cannot risk editing anything in the Open storage file structure except via the FM interface.
Whilst the new FM 12 containers and the external storage features are pretty cool, I have not adopted them in many solutions for that reason.
My preference is to put the images elsewhere (eg a web server) and calculate their path to display...