You can use Windows Disk Management to examine how the space on your 2 TB unit is allocated to Partitions. Within Windows click on Start in lower left and in that menu RIGHT-click on My computer and choose Manage. This opens the Computer Management window. In its left pane expand Storage if necessary to choose Disk Management. There will be two scrolling windows on the right. The upper one shows all the drives Windows can use right now with their names, Capacities and Free Space. Below that are the hardware devices represented by horizontal blocks. There will be one there representing your 2 TB unit with a label block to the left showing it as something like "Disk_0" with its size and a few details. To the right will be one block representing the portion of that unit already assigned to a Partition that is the named drive you are using for storage. That block MAY be the full space of the unit, which would indicate that the drive ought to have about 1.85 TB of Capacity. But my guess is that it does NOT take up all the disk's space, and to its right will be another block labeled "Unallocated Space". This would indicate that, at the time the first Partition was created and formatted, not all of the available space was assigned to that Partition. The Unallocated Space could be used two ways.
The simple possibility is that you can create another Partition from all of the remaining Unallocated Space, then format it and use it as a completely separate Drive with its own new letter name. But if you want to have ALL of the 12 TB drive used as one large drive Volume, that is not the way.
The second alternative is to Expand the existing Partition to take in (add) the Unallocated Space. You should be able to do that since the first (existing) Partition is not your C: drive from which you boot. You may be able to do this by RIGHT-clicking on the existing Partition to find a menu choice to Expand the Partition, and then setting it to use up all available Unallocated Space. But if there is no such choice, you can do it using third-party software like Partition Manager (you must buy it) or some freeware. Be aware, however, that there are a few tricks to doing this, and you'll want a software utility that can handle the whole job. One of the problems I've seen here is someone does the Expand operation in some manner, but the Disk File System on the existing Partition seems unaware that it has been expanded and still fails to show or use the new full capacity.
So, check the layout of the disk unit. If it has Unallocated Space, you can decide how to use it. On the other hand, if the existing Partition does already include all 1.86 TB but there still is 500 GB "missing" in the drive, then there must be some very large hidden files on it.