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#11
I'd install Partition Wizard Free.
Then I'd use it to "convert MBR to GPT".
As was previously stated, MBR partitioning can only support drives and partitions up to 2TB, and a max of 4 "primary" partitions. Or, 3 primary partitions and the fourth used as an "extended partition" inside of which any number of "logical" partitions can be allocated. This approach (using "logical" partitions) is how you used to have to go if you wanted more than four partitions on your drive. Note that you can only boot from a "primary" partition (i.e. the Boot Manager "system reserved" active partition, in an MBR setup). You can't boot from a "logical" partition.
That all goes away if you use GPT partitioning. No more 2TB limit (i.e. you can support drives larger than 2TB, and you can have partitions larger than 2TB). No more four partition limit. There's no such thing as "primary" or "logical" or "active', etc.
You're running Win10. There cannot be any reason why you can't partition your drive using GPT, and arrange as many GPT partitions as you want of any size (use Partition Wizard to do all of this), and have it be your boot drive.
Your 2TB story is because the 6TB drive is currently partitioned as MBR, and only 2TB of it is visible through MBR. It just needs to be converted to GPT with Partition Wizard and your problems are solved.
Note that Win7 also supports GPT partitioning, and booting to a drive partitioned with GPT.