Hi, @LoolyJar
Thank you for posting in Microsoft Q&A forum.
Possible cause: removable drive, not an internal fixed drive.
This looks like MDT was not finding the internal drive, but did find the USB drive when running DISK PART.
You may check rylar's answer in the thread to see if it helps:
https://community.spiceworks.com/topic/2318382-mdt-failure-7712
(Please Note: Since the website is not hosted by Microsoft, just for your reference.)
If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Add comment".