Many problem on Mega 6.3 GT-i9205 after rooting After rooting my phone, (Samsung Galaxy Mega 6.3 GT-I9205), I can't turn my WiFi back on but the important part of it is that my device storage is totally empty. It causes some apps/games to force close because all the files are gone, however, my rooting succeed... 

Look at this image, it shows that my memory suddenly becoming very low after rooting!
From the thread you linked to (highlighting my own): 


  The root via vcore doesn't work any more. Please get it looked up. Lots of people online are getting this problem as they flash the root. Problems are all of the internal storage are gone, left with 8gb and all apps in storage wont work.


Which seems to be the issue you are having. This appears to be that this root method does not work with the newer, Knox enabled bootloaders.

This method is the new way to root the device. 

Right now, your best bet is to go to SamMobile and get the stock ROM for your device, restore back to stock VIA Odin, and try re-rooting, once you confirm your WiFi is working and your internal storage is back to normal.

It's always worth reading into a thread on XDA before attempting anything, to get a feel for what issues people might be having, and if there are any work arounds. I always put a big "READ, READ, READ" in my advice for rooting, as it can (and indeed does) go wrong and is not a nice feeling. Restoring to stock appears to work, and is mentioned in the thread you provided. Good luck fixing it and we'll be here to help if you need it.