SD Card as a Network drive in Wi-Fi Network?
A good application to do that is the Samba Filesharing. It shares your sdcard, making it easy to transfer files from/to your phone.
Settings
After installing the application from Google Play, open it and fill a password by touching the password menu. The default username is SDCARD and the default Workgroup is WORKGROUP. You can leave it that way or change it in a way that suits you.

I'd recommend enabling and selecting your WiFi in the WiFi White List . That way it only enables Samba sharing when connected to your preferred Network.

That done, you only have to go to your Network Shares and look for a device called ANDROID.
Login

Name: SDCARD
pw: (the one you defined earlier)

Open the shared folder and it will ask again for credentials, and you're done.
Here you have a video that completes this information.