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After KB4577062 is installed on Windows 10 Version 1909 any attempt to upgrade to 20H1 - either via setup.exe from the 20H1 iso or via Upgrade Assistant - will fail. Also the 20H1 Feature Update won't be offered anymore.

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This claim is supported by observation and here is the technical explanation:

The latest appraiser compatibility data is located in the file Windows\appcompat\appraiser\Appraiser_AlternateData.cab inside the cab you will find the system compatibility shim database appraiser.sdb.

This shim can be inspected by first decompressing it via SdbUnpacker and by converting it in turn to xml using sdb2xml.exe.

Now we see the following upgrade block definition in the human readable xml data:

    <MATCHING_INFO_BLOCK>
      <NAME type="xs:string"></NAME>
      <APP_NAME type="xs:string">2009C Manifest Regression</APP_NAME>
      ...
      <MATCHING_REG>
        <NAME type="xs:string">SOFTWARE\Microsoft\Windows NT\CurrentVersion</NAME>
        <REG_VALUE_NAME type="xs:string">CurrentBuildNumber</REG_VALUE_NAME>
        <REG_VALUE_TYPE type="xs:int">1</REG_VALUE_TYPE>
        <REG_VALUE_DATA_SZ type="xs:string">18363</REG_VALUE_DATA_SZ>
      </MATCHING_REG>
      <MATCHING_REG>
        <NAME type="xs:string">SOFTWARE\Microsoft\Windows NT\CurrentVersion</NAME>
        <REG_VALUE_NAME type="xs:string">UBR</REG_VALUE_NAME>
        <REG_VALUE_TYPE type="xs:int">4</REG_VALUE_TYPE>
        <REG_VALUE_DATA_DWORD type="xs:int">1110</REG_VALUE_DATA_DWORD>
      </MATCHING_REG>
      <BLOCK_UPGRADE>
        <BLOCK_UPGRADE_TYPE type="xs:int">3</BLOCK_UPGRADE_TYPE>
        ...
      </BLOCK_UPGRADE>
    </MATCHING_INFO_BLOCK>

This basically tells the system that the upgrade is to be blocked if the Windows Version is 18363.1110, which is the case if KB4577062 was installed.

After uninstalling KB4577062 the upgrade is possible again.

Does anyone know if this is documented or why Microsoft has globally blocked all fully patched Windows 1909 machines from upgrading to 20H1 ?

gollum
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    It’s impossible to determine the reason your machine is blocked from receiving 2004 based on the information you have provided. It probably has to do with the fact the patch is a Preview patch. – Ramhound Oct 08 '20 at 14:03
  • " why Microsoft has globally blocked all fully patched Windows 1909 machines from upgrading to 20H1 ?" Maybe ask Microsoft? – Moab Oct 08 '20 at 16:09
  • @Ramhound This is not merely my machine since the latest system compatibility shim is the same for everyone. So this is true for any Windows 1909 with this KB out there ;) However, the thing with the preview patch might be true. – gollum Oct 08 '20 at 17:02
  • Is this the question? _Does anyone know if this is documented or why Microsoft has globally blocked all fully patched Windows 1909 machines from upgrading to 20H1?_ If it is, then what I can tell you is that I have a machine next to me that I have recently upgraded from 1909 to 2004 and eventually to 20H1. I cannot see here any global blockage done by Microsoft solely through a Microsoft Windows software update package. – DOBRESCU_Mihai Oct 09 '20 at 15:00
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    @MihaiDobrescu yes. Which 1909 build number did you have, 18363.1110 or lower? – gollum Oct 10 '20 at 06:34
  • Good question, @gollum. See, this is why I enjoy talking to complete strangers who are lingering out there, on the Internet. Either I get into incredible complications, or I learn some valuable lessons. In this case, it is an "A.S.U.S." "X551MA-SX056D" Laptop Personal Computer described at the https://www.ASUS.Com/Laptops/X551MA/Specifications/ U.R.L. – DOBRESCU_Mihai Oct 12 '20 at 06:45
  • A quick check of its recent software update process has revealed the following stages: "Microsoft" "Windows" "10" "April Update" Version "1803" Code-Named "RedStone", Operating System Build "17134.523"; "Microsoft" "Windows" "10" Version "1903" "May 2019 Update" Code-Named "19H1", Operating System Build "18362.295"; "Microsoft" "Windows" "10" Version "1909" "November 2019 Update" Code-Named "19H2", Operating System Build "10.0.18363.836"; Microsoft Windows 10 Home Edition Non-N Version 2004 May 2,020 Update Code-Named "20H1", Operating System Build 10.0.19041.388. – DOBRESCU_Mihai Oct 12 '20 at 06:45
  • So the answer to your question is: lower. Cheers! – DOBRESCU_Mihai Oct 12 '20 at 06:45
  • There are two notes that I still have to add to my previous comments, @gollum, on this question: first of all, in the sentence "_I have recently upgraded from 1909 to 2004 and eventually to 20H1_", actually there has been only one software upgrade process, as 1909 _is_ 19H1 and 2004 is _20H1_. – DOBRESCU_Mihai Oct 12 '20 at 07:27
  • Also, the build number that is written above, 10.0.18363.836, might not be the latest build number that was present on the machine before the upgrade to 2004/20H1. It is simply a build number that I jotted down once, for my own reference. So I do not really know if this machine can be put inside the category that you call as _fully patched Windows 1909 machine with latest build 18363.1110_. – DOBRESCU_Mihai Oct 12 '20 at 07:29

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