Wow it was even easier then I thought.
Again, this is with Windows 8 (and works with every version of Windows 8 and I'm 99% sure it's done the same way with earlier Windows).
While loged in with an admin user account, right-clicking on the partition(s) and going into :
Properties - Security - Selecting 'Users' from the box - Edit - Unchecking the 3 values that are listed by default (which are: Read & Execute, List folder contents, Read) - Apply.
After applying, 'Users' will be removed the from top section and BLAM!, non-admin accounts will no longer be able to access the partitions.
Agreed, the partitions are still listed for non-admin user accounts but will be unavailable / unnaccessible for them.
No need of a 3rd party software, no need of Group Policy.