The Problem: everything in your Windows 7 32-bit to Windows 10 64-bit wipe and load/refresh task sequence works (pretty much) flawlessly, but upon getting to Windows 10 64-bit and attempting to convert to GPT, you notice that Windows is loaded on your D: drive, not your C: drive, or you can’t find any volumes at all.
Documentation: Microsoft has a great article detailing the conversion from MBR to GPT on their docs website. In the table right below the video, they detail some common things you can do with MBR2GPT.
The second item in the list is a bit more meaningful and your success in successfully implementing a Win 7 32-bit to Windows 10 64-bit refresh (wipe and load) task sequence depends on understanding this bullet point:
Convert an MBR disk with BitLocker-encrypted volumes as long as protection has been suspended. To resume BitLocker after conversion, you will need to delete the existing protectors and recreate them.
Because this is a wipe and load/refresh scenario, we know we have to suspend Bitlocker to get the task sequence cooking successfully. Mike Terrill blogged about this and presented on it a couple of years back. Microsoft also has good supporting material over at docs.
So we will go with the assumption that you have found your favorite blog to suspend Bitlocker before you have started the process. (Hint: if you do not use Bitlocker, you should enable it in your Windows 10 task sequence).
Proposed Resolution: Here are the commands to delete your existing tpm protectors and recreating them:
manage-bde -protectors -delete c: -type tpm
manage-bde -protectors -add c: tpm
You should be able to create two task sequence command-line objects to successfully get you to Windows 10 64-bit.