It is now November 2020 and I have been struggling with the Win 10 2004 update for many months. Continuously getting No boot drive found - 3F0. I have tried everything suggested by virtually everyone; but to no avail.
It then dawned on me after looking at my boot order in my BIOS. I had 5 DISABLED entries for Avast ! GrimeFighter - a product which I haven't used since my terrible experiences in 2015.
Funny thing is I had successfully installed and upgraded my Win 10 from the start without many issues (that weren't caused by Microsoft that is). This one was a real head scratcher.
I tried all sorts of remedies moving the GrimeFighter below the active devices, resetting my BIOS, many windows based solutions and so on. After awhile I noticed after every failure to boot, one of the GrimeFighter entries would always be the top location to boot loader list - but it remaining DISABLED.
Finally after many many attempts to install the updates (20+) I decided to try to remove the entries from my BIOS. I ended up finding Visual BCDEdit on the web. My thoughts were that for some reason Windows or some other process was not checking the state of the loader and was just using the top entry to boot - a loader which hasn't been there for 5 years.
Using Visual BCDEdit, I was able to delete the entries entirely. Another upgrade attempt and viola, it worked.