Robbie,
I am just curious, if you were to carve out some space for a new VM running, let's say Ubuntu, what would Ubuntu see? For some strange reasons, I have a feeling it is the OS. If vSphere is reporting it as 7.xTB, if RDM was done properly, it should be visible. However, if the OS never understood anything above a certain size in the HDD.... maybe that's where the issue is.
When you say VMWare 6.5, I am assuming ESXi 6.5, correct? Also, here is a thread I found that have similar symptoms to what you are seeing...
http://www.tomshardware.com/answers/id-3037971/seagate-8tb-hdd-shows-1307-91gb-win7-2tb-bios.html
The op also saw 1.33TB from a 8TB Seagate.... One of the answers was, switch channels.