Z200/Z400/Z600/Z800 Windows 11 Boot Failure Nov 2025 KB5066835 - A Disk Read Error Occurred

[vanadu]

Zx00 (Z200/Z400/Z600/Z800) fail to boot after upgrading or clean installing Windows 11. A black screen with the 'A disk read error occurred - press ctrl shift delete to restart' is displayed. The boot failure appeared after the Windows 11 KB5066835 upgrade. As of 11/15/2025 there are two threads on this topic but this is the only one that has a useful discussion - the other one only has a link to the one below:

https://h30434.www3.hp.com/t5/Business-PCs-Workstations-and-Point-of-Sale-Systems/HP-Z800-killed-wit...

This thread includes two recommended fixes. One by Akierum entails overwriting the 100MB boot partition of the broken W11 build with a pre-KB5066835. This only works if you have a backup of the pre-KB5066835 build. The other by SDH entails a clean W11 install that excludes the offending W11 updates. Neither of these solutions help Zx00 users wanting to do an in-place W11 upgrade over their W10 install who do not have a backup of an existing W11 build prior to the Nov 11 W11 updates. I'm creating this thread because the title is more comprehensive and the accepted solution for the earlier thread is only applicable in case where the user has a complete backup of their in-place W11 install prior to the 11/11/2025 updates.

[vanadu]

I need to attempt a fix for this with an in-place W11 upgrade, not the clean install SDH details. I just can't take the time to set up W11 fresh at this time. Right now, I've restored my W10 build from the clone I made prior to the W11 upgrade. I made that in-place upgrade using the W11 25H2 ISO I got here: https://dongknows.com/steps-for-windows-11-upgrade-on-unsupported-hardware/. This W11 in-place upgrade worked perfectly until 11/11/2025 when MS pushed the KB5066835 update. My plan is to do the in-place upgrade with the W11 ISO and Rufus settings SDH details here: https://h30434.www3.hp.com/t5/Business-PCs-Workstations-and-Point-of-Sale-Systems/HP-Z800-killed-wit.... 

Capitan_baudo, I'm wondering if you're in a similar situation:  you need to do an in-place upgrade and have no backup of your working W11 upgrade prior to the KB5066835 from 11/11/2025. 

[SDH · Accepted solution]

Vanadu,

Maybe I don't fully understand what you've already tried. What I'd do:

1. Clone your W10 Pro 64 build from your image.

2. Update that with the latest W10 Pro 64 updates Windows Update has go offer. That will still work, it is just newer W10 Pro 64 updates that won't be offered (unless you pay, or otherwise, get them).

3a. Add a folder to your desktop containing the Microsoft wushowhide.diagcab utility (for fast access).

3b. Get you BIOS settings to factory defaults.

4. Make sure in Settings/ Windows Update you have the option of "Get the latest updates as soon as they're available" turned off.

5. At this stage I open the Properties window of my boot drive onto desktop and then run Disk Cleanup both ways (click on "Clean up system files" and after that is done use the More Options tab/ System Restore and Shadow Copies/ Clean up). Then I run the Error Checking and Optimize and defragment drive (the two tools available from the open Properties window's upper "Tools" tab.  I finally use elevated CMD to run the sequence of:

DISM.exe /Online /Cleanup-image /Restorehealth
sfc /scannow
chkdsk C: /f

If you add some returns below the last one and copy/paste all that (including the returns) into the elevated CMD window it should autorun all 3 automatically in sequence. When that is done answer Y to the chkdsk question, exit CMD, restart, and you've got a pretty clean machine.

6. Now disconnect from the internet.

7. Cold boot, insert your Rufus 4.11 thumb drive created as I show below and describe in more detail in the other main post to produce a MBR-Legacy BIOS W11 25H2 type of Rufus thumb drive, navigate inside that drive, double click on the Setup.exe in that. That should allow an in-place upgrade your W10 in the standard way. Shutdown, cold boot (still disconnected from net) to confirm you are now running W11 25H2.

8. Get ready to use Wuhideshow.cab, reattach to internet, confirm you're connected, launch the .cab, run it in its hide capability, it should see available W11 25H2 updates just like in the main post, hide that one, don't forget to then run Windows Update to install the others that were provided.

Just "hide" that bottom one...

The GPT/UEFI settings are different and for a second thumb drive...

I believe this will work for you.

[vanadu]

Thanks, SDH. I haven't done anything yet, but thanks for adding the procedure for the in-place upgrade. When I do I will follow your procedure to the letter and let you know how it works out. I'd also recommend that Capitan_baudo do the same. I should be able to tackle it in the next couple days and post to this thread about how it works out.

[capitan_baudo]

Hi everyone,

Here I am, I've read the previous two posts. I think SDH has well-expressed the concept to try, the possible sequence to try and achieve a good result.

To respond to Vanadu (I thank Vanadu again for opening this post), I have several options, but I don't want to upgrade from Win10. I've tried a clean install of Win11 several times. I have several Z400s available and one Z600, so it's not a problem to try again to find an acceptable solution. The last attempts I had made before reading the various posts had failed, and I even thought the Z400 might be defective in some way 😕

Tomorrow morning, the first test I'd like to do is to enlarge the famous 100MB partition to 500MB or 640MB. As a first attempt, considering that I have three SSDs locked up with Win11 that no longer boots, I was even convinced those SSDs were defective!

Then, as a second attempt, I'd follow the instructions in SDH's post to try and start over from scratch to get to a stable Win11 installation.

It remains to be seen whether subsequent updates will not change anything and whether Win11 will remain functional.

One more thing: I have another Z400 with Win11 (upgraded from Win10 a few months ago) and I'm terrified of rebooting after the updates for fear it might crash. This one is in production! I might replace it with a little time, though.

Well, I'll have a lot of work to do tomorrow and will post the results here as I come up with something useful.

My hope is that others will join the discussion and contribute, as these workstations are truly great, super-robust machines. It's too early to retire them, considering they have Xenon processors.

[SDH]

Make sure to learn how to use DiskPart. I saw multiple times when my Rufus 4.11 USB installer could not work with the target SSD until I used these commands after opening an elevated CMD prompt. That fixed whatever odd corruption that was holding up progress:

diskpart

list disk

select disk X (with X representing the actual correct disk number of the SSD)

clean

convert mbr

exit

That whole process takes less than a minute. I too thought I had bad hardware or some CMOS corruption... it was instead a piece of bad software from Microsoft. Their engineers have helped us many times in the past, and I'm happy to return a small favor by identifying the one update that seems to be the cause.

[vanadu]

Thanks again SDH. I used diskpart to inspect the bad W11 but didn't think to use clean / convert mbr. Capitan, you wrote:  "I'm terrified of rebooting after the updates for fear it might crash." I'd say it's 99% sure it will not boot. I'd suggest booting from a USB to a command prompt and using diskpart like SDH said. Who knows? I've been using the Macrium free USB tool with Rufus to boot the the command prompt. Macrium USB also has a 'fix boot record' option that helped me boot to W10 a couple times when the boot failed and displayed the Recovery screen with an error. But now that I know about diskpart clean / convert mbr I'd always try that first.

Capitan, did you make a backup of any of your W10 installations before the W11 upgrade? 

[capitan_baudo]

Hi Vanadu,

   I just got back from another place this morning where I had some issues to resolve, and right there, that magnificent Z400 continues to work perfectly, even after being upgraded to Win 11 a few months ago. The system I used, and have been using for some time now for other updates, was from CMD with elevated privileges:

   - setup /product server

This command, run on a clean ISO, as you well know, is used to perform an in-place update in a mode that tricks the operating system and bypasses controls like TPM and processor requirements, which is not officially supported. This has saved the lives of several machines no longer officially supported by Win 11 but perfectly capable of operating with Win 11. The question remains of how the updates will behave over time, but for now they're working.

The updated Z400 turns off and on just fine. A few weeks ago, I ran several updates on it, since the boot issue (A Disk Read Error Occurred) was still little known. This morning I didn't have the courage to run the latest updates because I was rushed with other work. I'll definitely do it tomorrow morning, but first I'll make sure to clone the disk to recover from unexpected crashes! Before running the updates,
I'll check whether the KB causing driver problems has already been installed or is listed, so I can get the most complete feedback possible. Unfortunately, I still haven't figured out which KB is actually causing the problem, or whether there's more than one. I'll have to take my time and investigate.

Now that I'm back at the office, I'm preparing to test resizing the boot partition on the other Z400 that's still displaying the error (A Disk Read Error Occurred), although I'm not sure this will yield good results.
I checked the disk with diskpart but I can't find any reference to GPT on the disk. When analyzing the partitions, I don't see "*" which should indicate a partition converted to GPT. So, should I assume the disk is still correctly formatted in MBR but with incorrectly sized partitions? On older PCs, like the Z400 or Z600, I've always installed ISO from USB sticks prepared in Rufus with an MBR system for BIOS legacy.  Is that what you meant to point out, or did I misunderstand? Thanks, Vanadu.

Then I'll try upgrading the Z600 from Win10 to Win11, writing down all the steps and seeing what happens. Yesterday I reinstalled Win10 on the Z600 and applied various patches, installed antivirus software, and made a few small customizations so the machine is similar to a production machine. At this point it is ready to attempt to upgrade to Win11

[capitan_baudo]

Hi SDH,
Thanks for your suggestion. Maybe I didn't understand what you meant to apply. The command sequence you wrote.

   diskpart

   list disk

   select disk X

   clean

   convert mbr

   exit

Do you run it before upgrading to Win11 even if the disk is already in MBR format, or something else?
Or do you want to apply it to the Win11 system that's already causing the error in your post? If it's already causing the error, are you considering unmounting the disk and mounting it as a secondary disk on a working PC, then analyzing the secondary disk's partitions as you suggested with diskpart?
Thanks for the clarification.

[vanadu]

Capitan - My W11 upgrade from W10 worked fine too until Tuesday 11/11/2025. On that day, MS pushed the KB5066835 update to W11 machines worldwide. If you read SDM's procedure carefully, he explains a way to upgrade to W11 and exclude the KB5066835 update that breaks our Zx00 machines. If your W11 systems still work, it's probably because you turned off automatic updates before 11/11/25 so the KB5066835 was never installed on your Zx00 machine. As soon as you install that KB5066835 update, your boot will break. So if you haven't installed that update yet, make sure you do not allow Windows to install any further updates. Turn off automatic updates in Settings, and also disable automatic updates in Services app or Group Policy Editor. You can google how to do that if necessary. If you have a working W11 system without the KB5066835 update, clone the HD to create a backup so you can restore it to that state. Then follow SDM's procedure to exclude the KB5066835 update from future updates. If that doesn't work as expected, at least you have the cloned backup without the KB5066835 update. That will be my plan moving forward. 

Also, I think the diskpart procedure should be performed on your W10 machine before upgrading to W11. I may have that wrong but that is my understanding. Good luck!