thanks for posting this hww; but this has been pretty widely known since the first week of August.
However, I might add that during a recent Win8.1-W10 upgrade on a client's netbook a couple of weeks ago; no one in my local computer club could figure out why the upgrade kept hanging and would never complete. After a couple of long days on my workbench, I finally figured it out. The guy's hp netbook had only a 30MB hard drive in it, and was formatted oddly so that there was only like 21.8GB of capacity on the drive. He had filled up his drive to 95% of capacity; and only had 5% free space available (about 1.43GB). As you may know, Windows requires at least 25% free space (no more than 75% used space) in order to do any kind of OS upgrade.
This was kind of embarrassing to me, since I almost always check this, but I jumped into thinking it was something else like a busted registry, virus, corrupted/missing windows systems file, etc. and started working on lots of other repairs after I backed up client's data. Skipped over looking at this until about 2am in the morning on the 2nday day I had the netbook, when the W10 installer told me my hard drive had insufficient space, and it needed a minimum of 9GB free space in order to complete the upgrade. After I cleaned off about 20 programs the client didn't need, and copied his data to DVD disc for later upload to OneDrive cloud; I was able to run through the upgrade no problem. Repaired netbook now has a reformatted drive with about 26% free space; he really can't use it for storage and that's ok since he has 2 other full size laptops plus various cloud backup accounts for that. He opted not to go with a larger hdd, which I offered to install; he decided to just use it for travel mode with web, email, and socail media access.
Moral of this story is the same I've been telling people here on the forum since July: "If your laptop or PC isn't running healthy (per Microsoft specs) prior to your Windows new OS upgrade, it either will fail the upgrade, with or without an error message code, or it will just hang or freeze".
Pretty basic stuff, but even us pros can sometimes miss this very obvious guideline.