The default SYSTEM User under windows will not be able to access mapped network drives. Services run under this user by default, like Apache.
Mapped network drives are mounted for a specific user and won't work for other users.
Tips:
- Instead of using the drive letter use the UNC path. (example: '\\server\share instead' of 'g:\')
- Mapped drive is mapped for some specific user, while service is running in another context and is not aware of this mapping. There is a privilege called "Log on as a service". The user account you specify for service should have this privilege. Use secpol.msc to grant it.
- Run the server from the local user to which the network drive is mapped, instead of running it as a system service. Use services.msc and change the user under the Logon tab.
Always do tip 1. Choose between tip 2 or 3.