Can't convert to GPT on a 4TB HDD

[regnissah]

So for a while I had been under the impression that 2TB of my 4TB Seagate Barracuda HDD was just unusable by Windows until I got hired at a tech repair shop where I found out/learned that the drive must have been initialized in the MBR file format. When I had initially setup the computer and installed Windows 10 Pro, my 250GB Samsung Evo 950 SSD and the Barracuda were both plugged in as it installed. At the time I had thought nothing of it. I built the PC in March 2017. I had made the 2TB partition that was available on the Barracuda and used it as a backup sort of deal, which just filled up to 1TB recently. It had a few apps installed and a large amount of games installed. I had an external drive with over a terabyte left available and I decided that I would finally convert my useless 2TB into usable by using diskpart and converting the drive to GPT. After I spent hours transferring data/games, I came to a crossroads when I found that diskpart came up to an error when cleaning it. The error reads that it is not able to be cleaned because it has parts that are being used on the system, even though I initially installed Windows on my SSD. I came to a post here: http://www.tomshardware.com/answers/id-2605599/convert-gpt.html#21247353
This post is pretty much my exact problem, and I proved that my system cannot boot without my 4TB being plugged in. The solution on that post says that somehow Windows built a partition on that drive that it cannot boot without. I would really like to be able to use all 4TB of that drive without clean installing Windows on the device.

 

[BFG-9000]

There are free tools that allow you to copy the boot partition to your system drive SSD. I used a bootable Minitool Partition Wizard free disc to do this long ago but as the current free version requires an install anyway, also look at EaseUS. In any case such a tool can easily delete partitions and convert a disk to GPT too, because it runs outside of Windows. Simply resize your SSD partition smaller so it can hold the boot partition, then clone the boot partition from the HDD to the SSD. Finally Set the boot partition as Active, and it should be bootable. Incredibly, EaseUS of China has much better English instructions than Canadian Minitool.

There is even a way to make the system partition also the boot partition at least for legacy BIOS--it involves putting a \boot folder in the root there. Unplug the HDD and boot from the Win 10 installer to repair the boot sector. This will write a folder called "boot" into your main SSD partition so you won't have or need a separate boot partition.

Let this be a lesson to never install Windows with more than one drive connected--the installer seems to decide at random what drive to put the boot partition on!