If you're sharing using as network path with UNC notation then I think that will only work with disks that are local to the Wowza installation. I suspect your best options are to run Wowza as a "normal" user and then you can reference the shares in StorageDir and Wowza should be able to have access to them. There's a quick and simple way, and a more involved but more robust method,
Option 1: Run Wowza in stand-alone Mode
This option allows Wowza to run as the user who is logged on to the Windows machine. Therefore the Wowza Streaming Engine services will have the same permissions as yourself and should be able to access the shared storage.
To run in Stand-alone mode you will need to stop the Wowza Streaming Engine service via the Windows Services manager and then open up a command shell and navigate to the following directory (assuming standard installation of version 4.0.x on your C: drive):
C:\Program Files (x86)\Wowza Media Systems\Wowza Streaming Engine 4.0.x\bin
You then type in the command startup.bat. This runs Wowza in stand-alone mode. You should then be able to reference shared drives in StorageDir, like Z: etc.
Option 2: Create a Windows Scheduled Task to run Wowza Streaming Engine as a named user
This option essentially results in the same outcome as option 1 but it executes startup.bat script at user login, thus automating the process. As with option 1 you will need to disable the Wowza Streaming Engine service if it is set to automatically start.
We also use this method to allow Wowza to perform hardware transcoding under Windows, and so the
following article has the relevant steps.