I guess I can't edit a post when it's too old. Anyway just in case anyone comes across this, I'm a dummy. Fast boot was ultimately the culprit due to how it caches/schedules writes. Had to disable it...

So I am dual-booting between two physical drives: A host drive with Windows10 1803 (SSD/GPT), and a host drive with Windows10 1607 (HDD/MBR). The 1803 is a fresh install via M$ Creation Tool, and the...