In order to access network storage you will need to edit the FTGate service to run under a user account as follows:
Create a new network user account in the name of FTGATE_SERVER
On the network machine providing the storage create a network share for the files called FTGATE_SPOOL
Go to the Service control panel on the FTGate machine and open the services control
Open the FTGate mail server service and change the startup details to specify the account FTGATE_SERVER as specified in 1
The service control panel will then modify the account settings to allow appropriate access
Start the FTGate Service and open WebAdmin
In Configuration, click System
In Safe Mode, click Safe Mode
In Spool Path, enter \\computername\ftgate_spool