CasualNick

So  ...  any standout should have been like 12,000MB of memory in use ... should have been pretty obvious.
Very few programs should have more than 1000MB of memory (more then 1-2 and you have a real problem).
 
Yes, it would be expected that a reboot would reset it to normal.
 
What you now need to look out for is:
1.  Does the memory used jump suddenly, if so what are the likely actions / triggers causing it.
2.  Does the memory used increase slowly, but constantly increase, not ever dropping much.   This is classic memory leak - harder to work out what is causing it.

Click on the "memory" column in Task Manager 'processes' to sort the column in decending order.
The largest memory user at top if its a program you recognise then patch / update it or uninstall it as appropriate.
If its not something you recognise, post the list (image) here for help.

Have you checked Disk Management in Windows? You may have to assign it a partition(s). 

EDIT: Also, the Storage Devices or Troubleshooting subforums would have been the more appropriate place to post this. 

Issue solved.
 
Right click on the Unallocated section and click New Simple Volume. Follow the onscreen prompts.