Phantom Disk that won't go away

Recently my boot times have gone to crap, with my 970 Evo 1TB taking a good 2-3 mins to load Windows. BIOS has been reset, and Samsung Magician benchmarks put this as working at expected speeds... When it actually gets going.

Ok, so maybe a previous restore point could work? Just keeps giving error messages and failing. I could live with that if it meant just reinstalling Windows, but I can't do that either. I've got a USB 2.0 Disk (yes, I know) as the installer and it takes a good 10-15 mins to even let you select language. After this it tells me Windows cannot be installed to this Disk as it needs to be GPT. I have no idea how this could happen as it would have to already be since it just had Windows on it. However diskpart won't load through command prompt about half the time, and when it does it freezes and never completes when I try to convert to GPT or format the drive.

But here's the weird part - it also says there's a Disk 1 connected with 3TB of unallocated space. While I have previously used a 3TB drive in this PC, I removed it a few weeks before having any issues (it's an Itx build so 3.5" drives aren't always practical). So there's no drives connected except the nvme SSD. When I try use diskpart to erase the Disk (it's unallocated so no partitions), it says it's unable to erase or format the Disk. Interestingly, when I remove the SSD and boot the installer the 3TB ghost drive is now Disk 0.

I'm guessing this phantom drive is the main cause of my problems, but I'm at a loss as to how to get rid of it, or how to convert the SSD to GPT so I can actually install Windows again.

I've done a bit of googling and haven't come across this exact issue before, any help getting my new PC to stop being a paperweight would be greatly appreciated!