1. The problem, of course, is that the System Reserved partition is located on the 6 TB secondary HDD rather than the 250 GB boot drive.
2. So as the situation now stands you need to have the 6 TB secondary HDD installed upon booting to the OS because of the location of the SR partition. As a consequence, you cannot initialize that drive to the GPT-partitioning scheme without deleting the SR partition containing data. (Disk Management will convert the disk to GPT only if NO data is contained on the drive. And the SR partition contains data, doesn't it?). But if you do that, i.e., delete the SR partition, the system won't boot as things currently stand. So you're caught in a kind of a Catch-22 situation. Capiche?
3. But there's a way to get around this. Delete the SR partition. You will need to utilize DiskPart to accomplish this; Disk Management will not carry out that deletion. The final command in DiskPart (after selecting the SR partition) is "delete partition override" (no quotes) <Enter>. You need the "override" flag (protocol) for this command.
4. At the Admin Command Prompt, type "bcdboot c:\windows /s c:" (no quotes) <Enter>
You should get a message that the boot files have been successfully transferred.
5. Using Disk Management, select the "Mark Partition as Active" option for the C: partition. (I'm assuming that drive has been MBR-partitioned.)
6. Review the commands you've invoked to ensure they're proper as per detailed above before you reboot the system.
7. You now should be able to boot directly to the Win 10 OS and if you haven't previously done so in step 2., initialize the 6 TB HDD to the GPT partitioning scheme and partition/format the drive as you wish.