This is how I would replace the default drive with a bigger size (3T~6T HDD). Get a big HDD and connect it to the XO by external USB3.0 port. It will ask if you would like to format it to be compatible with the XO. Choose yes and format. Unplug the HDD from the XO. Open up your XO, take out the default HDD, replace that with the formatted HDD, close your XO and done.
Logically, this would do the trick but I'm not sure. :-)