My Windows 10 Version 20H2 will not install the update 0x80004005.

I have researched and fixed my own with the two-step process.

Step 1:  Find out if your "Windows" system drive is GPT or not via Check if Disk is MBR or GPT in Windows | Tutorials (tenforums.com)

 -- if your Windows drive is MBR, you will need to convert it to GPT.  Google is your friend for this.

Step 2:  If all drives look GPT and your still not getting the 'upgrade assistant' (https://www.microsoft.com/software-download/windows10) to let you start 20H2 update, than you are like me, where the GPT 'table' (?) gets hosed up and you have to boot into 'safe mode' command line to run "sfc" with special parameters.  There's a thread on that: How to Fix a No Bootable Device Error on Windows 10 (helpdeskgeek.com)  Look for the section "Scan for Corrupt System Files Using SFC" and pay attention to the drive letters.  In this case there is SOMETHING that re-assigned drive letters to UEFI and you need to fix that with the steps they show for using the option /offwindir and /offbootdir to set YOUR correct drive letter.

No re-install required if this is really the root cause.

Good luck.

Thanks man I'm going to do this as well. What do i need tondo the NAS backups and so. I dont want to lose everything andrreinstall all my programs. I have a lot of programs work related. Not even thinking about all my games. 

well my friend, 

reinstalling stuff is a dead cert with my method, can't be avoided!

all i know is that i have external back ups of all my music, photos, vids etc, to a WD NAS                                                     

easy to put back in place after fresh install.

I'm not sure about programs, i just reinstalled from their respective websites, tried desperately to remember the logins then dragged n dropped the content from the external drive to the appropriate folder in the new comp drive.

would suggest you do a full back up of all your content to an external hard drive before removing anything.

as for your game data.....you'd need to reach out to someone else who knows , i don't have a clue!

I'm onto Motorhead and tequilla now, you're lucky i can still type.....

good luck!

Thanks man I'm going to do this as well. What do i need tondo the NAS backups and so. I dont want to lose everything andrreinstall all my programs. I have a lot of programs work related. Not even thinking about all my games.

Cracked it sunshine!!!

my hard drive finally failed, signs were there for a few months.

Changed 3 Tb Seagate 3.5 Sata for a 4Tb Samsung SSD, created USB Windows installer on lappy and started afresh.

nice new copy of windows, now fully up to date!

currently re-installing all the peripherals with loud Janes addiction, beer and smokes!

nothing lost thanks to NAS back ups.

Computer has a new lease of life, fast as ****!!!

Turns out that the issue with my failing hard drive was preventing my update but wasn't flagged or reported.

I am kinda guessing here, but i had tried everything through this thread and nothing worked.

If this is the case and the update failed due to hard disk issues, why isn't there an error code to reflect hardware issues??

I hope this can help someone else through the troubled update, maybe you should run a disk scan just in case (didn't help me tho, all came back good, then it died.....the signs were there........you know when you know....)

Oh well, onto Red Hot Chilli's and Bourbon N dry now. The sense of relief is intoxicating!

I agree with you. I am totally frustrated and have given up. I guess I will live with 1909. Very disappointed. :(