Windows 10 Partition Setup

I have had UEFI on my system for a long time under Windows 10. Microsoft made change to force Secure Boot on Windows 10 that my computer has no support for. Now I can no longer use UEFI with Windows 10, because my computer will not boot successfully. I cannot disable Secure Boot in my BIOS, and Microsoft make a call via Secure Boot that crashes my computer immediately at startup. Microsoft will not fix this, so I just installed Windows 10 with a BIOS MBR boot instead. I know there is a limit of 2TB as the maximum disk size, and my C: drive has a partition set as this size. My total disk space is 5.46TB. What must I do to create additional partitions? Right now I have 3.46TB of unused space. The Windows 10 setup would not allow me to create additional partitions (I do not remember it being this difficult with older versions of Windows). I cannot extend or create secondary partitions at all.

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I got this issue resolved. The short answer is that Microsoft provides zero ways to do this with Windows 10 alone. You can only do it using a custom non-Microsoft boot loader. However, you risk future breakage going this route, because Microsoft is rather reckless with future change.
It turns out that my own hardware has support for this in BIOS mode. I simply wiped the large single partition and created three virtual partitions:
*2048G BOOT
*2048G DATA
*14XXG TEMP

I reinstalled the latest Windows 10 via BIOS boot. It's RAID10 cached, so performance is still fast.