Yep. Rebooting to reinstall my motherboard chipset drivers AGAIN, same behavior. That drive had become increasingly flaky the last few weeks, and I never turn my computer off if I can help it. My guess is that at some point it bit the dust quietly, and upon boot, windows is running checkdisk without asking if I want to, except it's bugging out and not telling me this for the entire 20+ minute scan. Letting it sit and spin again to confirm, then I'll have to figure out which one's dead (I've got two ancient, identical 640gb drives in there for backup, so figuring out which one corresponds to E: will be fun) and unplug it.
So delightfully stupid
disabling dead drive immediately fixes boot issues. I really wish Windows would actually report this.
Also, now installed AHCI and Rapid Storage and chipset drivers from mobo manufacturer and USB 3.0 hard-drives still aren't seen by Windows 10 anymore, ever since the Creator's Update broke them a month ago.
Fuck and shit, bro. I'm gonna go eat my feelings instead of fucking with this nonsense anymore. @LMN8R all fury and rage aside on my part, if you have any advice on how to make Win10 see USB 3.0 hard drives again (they work fine on USB 2.0, and other USB 3.0 devices work just fine), I'd love to know :(