Answer
In order to be able to gain access to a network share, the WWW service must have access rights to the share.
To do this, set the GMS Configuration Service to logon as an administrator rather than using the system account. Providing the Administrator has read/write access to the share then you will be able to use this through GMS. Go to Control Panel > Services and select the GMS Configuration Server from the list of available services.
Click on the Startup button and under the "Log on as" area select the "This Account" option. Either enter an NT User to log on as or click on the button to the right and browse for the user. Enter the password for this user, and then enter it a second time to confirm that it has been entered correctly.
Now back in the GMS GUI you will be able to enter the full share name, directory path and file name to point to the location you would like to save the setup.txt file.
Note: this does not give you a fail-over solution, but does mean you will have a copy of the setup file (without mailboxes) for a new installation should one be needed in the future.
Keywords:setup.txt recovery share security