Hello cam
As mentioned, an MBR partitioned disk can only be used up to 2TBs. You can try that way or you can try another easier way. Convert the MBR disk to GPT disk with freeware, AOMEI Partition Assistant Standard. You can create almost unlimited (in fact up to128) partitions on an GPT hard drive. You can convert the data partition (not C drive) from MBR to GPT disk style with that freeware. But if you want to convert the entire disk, you may need the paid version. Yet in your circumstance, I guess the free version is enough.
If you do not want to convert disk style, I suggest you shrink the unallocated space into two parts, one part shoud be more than 1 TB. Then leave the 1 TB alone and make use of the rest. The software I mentioned allows you to merge unallocated space. Maybe you can have a try.
Regards.