I don't understand. Your Disks screenshots (here Zorin 17 install only using 256Gb of HD space - #9 by windswords) are saying the drive is a HFS250G32TND-N1A2A.
A websearch for that dive finds this: Amazon.com
The Spec on Amazon is confiusing as it says Density: 120GB/250GB/500GB
but is listed as a 250GB device.
From the other thread:
You are saying when you had Win7 on that same drive, it was showing the full 500GB?