First, open the Google Play store and navigate to My Apps. Check to make sure there isn't a pending update for Plex Media Server. If there is, install the update.
Second, go to Settings > Device > Storage & reset > SHIELD storage access > Over local network, to turn this feature on.
You'll be presented with a user name and preset password that you can use on a networked PC to access SHIELD folders. Make sure your PC is on the same network as your SHIELD.
On a Windows 7 (or later) PC, open Windows Explorer and select Network. You'll see your SHIELD listed under "Computer" using the device name (default is "SHIELD"). It can take some time for computers to be shown on the network. Try refreshing the Network page and restarting your PC if needed. To immediately connect to your SHIELD, type "\\SHIELD" into the Windows Explorer address bar. If you changed your device name (found in Settings > About on your SHIELD), replace "SHIELD" with your device name (without any spaces). Alternatively, you can type your SHIELD's IP address into the Windows Explorer address bar: \\192.168.1.X (find your IP address on the "Access SHIELD folders on PC" page).
You can also permanently map your SHIELD as a network drive by right-clicking on "Network" in File Explorer and selecting "Map Network Drive". Select an unused Drive letter. In the Folder field, type your IP address and the folder path to map. Example: "\\192.168.1.5\internal". Select "Reconnect at logon" and then Click "Finish".
SHIELD requires that your PC support NTLMv2 authentication. Visit Microsoft support for more information.
If you get a "permission denied" message, check to make sure that your username or password didn't change. If you need to re-enter a new password in Windows, delete the old password by opening a command window (type "cmd" in the Windows Menu Bar and select cmd.exe). In the command window, type "net use \\SHIELD /delete".
Note: You don't need to set up a Plex Media Server to use this feature.