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

[capitan_baudo]

Hi Vanadu - Thanks for the your correction. I wasn't clear on the part about diskpart either, as it can't be applied until after the upgrade. I also suspect it needs to be done that way. Thanks for your advice and the contributions of everyone else in the topic.

Regarding a Z600 the Win10 to Win11 upgrade procedure, I'm currently making backups before following the SDM procedure, which is well-described and detailed, as you mentioned, to prevent further errors. If possible, I'll also clone the original disk before the upgrade to speed up recovery, if necessary.

I'm now proceeding with the tests and will post again here.

[SDH]

Regarding use of DiskPart... you'd not use that sequence of commands for an in-place upgrade from W10 to W11 because that would fully erase your W10 build. It is a good set of commands to fully erase any OS build that is on your intended boot drive in preparation for a clean OS install, and also to convert your drive from GPT to MBR partitioning or from MBR to GPT. A GPT-partitioned drive cannot be used as a boot drive in these ZX00 pre-UEFI workstations. 

Another circumstance to use that set of commands is if the intended boot drive somehow gets corrupted and can't be seen by the W11 25H2 Rufus installer thumb drive. In such a case the fix is to wipe it clean and re-partition it to MBR type. Then you can do the clean install.

You may not know that DiskPart has many capabilities, but that is a topic for another time.

Also, it is easy to see from within your OS whether a drive is GPT or MBR partitioned. You get to that by using Disk Management/ click on the far left Disk 0 or whatever Disk number you're interested in, drag down to Properties, go up to click on the Properties "Volume" tab, and under that the "Partition style" is shown.

[vanadu]

Oh boy.. SDH thanks for clarifying that little detail about that sequence of diskpart commands wiping the W10 build. That would have been an unpleasant surprise for me! 

[capitan_baudo]

Thanks SDH, I didn't mean to use diskpart this way; that's definitely the wrong way, and your clarification reinforces my point.

I've checked several Win10 installations, and the partitions are very different. Some have 50MB of reserved space, others around 600MB. I'd say the average is 600MB. However, if I look at a Win11 partition, the reserved space is typically 100MB on a clean PC, or even 200MB on a VM installed from scratch without an upgrade (this one has a GPT volume). I've also checked other PCs built with Win11, and their hidden partitions are typically 100MB, always in GPT format, since the PCs are significantly newer and support UEFI well.
What I don't understand is the issue with the small partition on Zxxx machines, since previous posts discussed enlarging a partition that was only 100MB and was supposed to be larger.
This point isn't clear enough..

[vanadu]

My W10 system partition is also 50MB. It was upgraded from Windows 7. ChatGPT says this - and btw I got the 0x800F0922 error a few days ago when I tried to fix my W10 by copying just the 50MB system partition from my backup W10 clone to my current W10 SSD using xcopy.

Why is the system partition on windows 10 only 50 mb?

On many Windows 10 systems, you’ll see a small “System” or “System Reserved” partition around 50 MB. That size looks tiny compared to the more common 100–550 MB partitions, so here’s why it happens:

Why the System Partition Can Be Only ~50 MB

Because your disk was originally partitioned by an older Windows version—often Windows 7 or early OEM factory images—where the default System Reserved partition size was 50–100 MB.

When Windows 10 later installed or upgraded, it didn’t resize or recreate the partition, so it simply kept using the small existing one.

More details:

✔ Windows 7 default

• System Reserved partition was 100 MB, and some OEMs made it even smaller (50 MB).

• It stores:

  • Boot Manager

  • Boot Configuration Database (BCD)

  • Files for BitLocker (if used)

Windows 8 / 10 default

• Microsoft increased the size to 350–550 MB because newer Windows versions place extra boot files there (WinRE components, language files, EFI data, etc.).

Why Windows does not enlarge it

Resizing system partitions automatically is risky; failure could make Windows unbootable. So Windows keeps whatever size existed initially—leading to 50 MB system partitions on older hardware or upgraded installs.

❗ Issues a 50 MB system partition can cause

• Feature updates may fail with errors like:

  • 0x800F0922

  • “Could not update System Reserved Partition”

• BitLocker may not be enabled due to lack of space.

Solutions (if you ever need them)

1. Resize the partition manually using Disk Management + third-party tools (Windows cannot resize it natively).

2. Move WinRE to a different partition (advanced).

3. Reinstall Windows with modern partition defaults (creates a 550 MB System partition).

[capitan_baudo]

Good afternoon everyone,

   As promised two days ago, I managed to perform a clean sequence of operations thanks to the suggestions of many of you who contributed to this post. I especially thank Vannadu and SDH for their valuable contributions to the discussion.
This sequence suggested by SDH has been personally tested and verified by me on an HP Z600. It concerns the Win11 upgrade performed with the original Microsoft ISO. I hope this will be useful to anyone experiencing the same difficulties we encountered.
I will later publish another experiment with a reduced and optimized original Microsoft ISO (ChrisTisusTechUtil) to reduce the impact of Win11 on PC resources, for anyone who would like to try it.

HP Z600 Win10 (installation and) upgrade --> Win11 throught Original Microsoft ISO: WORKING NOW!

PRECISE: On this Z600, I configured four SATA hard drives in Raid5 mode, creating a volume that I will later use as a backup container, similar to a NAS. A fifth SSD will contain the sys-op installation.

1) The PC is connected to the network/internet; this is irrelevant at this stage. Clean install of Win10 22H2 from MBR ISO (Legacy BIOS) and automatic system updates, all those that can still be downloaded, restart the PC and test stability. Antivirus and small mods were installed; restart and test stability.

NOTE: The USB stick can be created with Rufus using commands already explained in previous posts.

NOTE. You can also start from an already functioning Win10 system that only needs to be upgraded; the essence of the initial operations remains the same.

NOTE for my configuration: The system (disk management) recognizes the Raid5 volume previously defined at the controller level.

2) Performed a backup with VEEAM Agent Backup Free on a network drive (I personally use this free program, which is excellent for both VMs and physical PC). You can also create a system image with your preferred program.

3) Optional: BIOS had already been factory reset in previous steps.

4) I copyed the Microsoft wushowhide.diagcab utility to the desktop.

5) I verified that the "Download the latest updates as they become available" option is disabled in Settings/Windows Update.

6) Check that the database is clean and the disk is free of errors:

        CMD with elevated privileges to run the following sequence:

        (first command is especially important if you are starting from a Win10 installation that has already undergone several updates over time, including crashes and failed reboots.)
   DISM.exe /Online /Cleanup-image /Restorehealth
   sfc /scannow-

        Optional if applicable --> chkdsk C: /f

7) Disconnect from the internet.

😎 Restart the PC; I access other Rufus USB stick I created previously (Win11 ISO 25H2 MBR, bypassing the RAM, processor, TPM, and other necessary settings; you can also do this by downloading the image on the fly with Rufus) --> I launch Setup.exe and run Win10 update --> Win11 -->  message warns that the update may not be compatible and may cause malfunctions on the PC; continue

9) Update complete. I reboot and verify that the system boots correctly --> it works and is stable. I notice the machine is quite slow compared to before and compared to a "light" installation I had previously performed. With subsequent reboots, it stabilizes better.

NOTE for my configuration: The system (disk management) recognizes the previously defined RAID 5 volume, which is already visible in Win10. This is very good; everything is working properly!

10) I reconnect the network/Internet; I run Wuhideshow.cab and select "hide updates":

--> I let the analysis run --> the first updates it shows me are consistent with the system:

I close the tool and call up the updates, verifying that they are the same ones the utility suggested --> I let them do their job and wait for them to complete:

Once the updates are complete, I restart my PC --> the PC restarts correctly.
I run Wuhideshow.cab a second time, which this time detects the KILLER update, and I choose to hide updates by checking KB5068861:

I close the tool again and run the system updates, verifying that they are the same ones the utility suggested. The KILLER update is NOT detected, and another one is detected. I let it run and wait for it to complete. I reboot and verify that the machine is stable.

The Z600 is currently stable and can be used.

11) End.. for now!

[vanadu]

Thanks Capitan_baudo for the excellent write-up. My Z600 machine is a dual-boot W10/Ubuntu and I've had issues with the grub bootloader that I need to fix first before I do the W11 upgrade. The first time I upgraded to W11, before the KB5066835 update, the dual-boot functionality was preserved and worked perfectly. Since KB5066835, though, there's been a lot of problems - even after restoring from the W10 clone for some reason broke the grub bootloader, which it didn't do before. I want to have the grub bootloader working again properly before I do the W11 upgrade. Also, I will add the additional step of cloning the SSD immediately after the W11 upgrade, before I reconnect to the internet, so that if at some point KB5066835 returns I can at least restore from the fresh W11 upgrade build. I should be ready to do the W11 upgrade as per SDH and your input today. I'll let you know how it goes, and thanks again for all your help!

[vanadu]

This is either good news or very terrible news. I'm not sure which. It appears MS has removed KB5066835 from the update queue -- at least it no longer appears in the wushowhide.diagcab update list as of this morning, 11/21/2025:

That might be good because they're reevaluating the entire KB, including whatever it was that bricked our Zx00 systems. Or it might be very very bad because they're fixing the issues that have been reported and are just going to brick our Zx00 systems in a later update. I'm afraid there's no way to know, and every time we install updates, we will have to hold our breaths. 

My approach will be to not install any updates. I turned updates off in Settings, Services and Group Policies, and I created a clone of my system immediately after the W11 in-place upgrade. I'll update the machine every 6 weeks and make a clone before every update. 

I don't know if there's a way to communicate this issue to Microsoft so they fix whatever it was that bricked our systems. I don't believe HP or MS are intentionally doing this. I don't think they care that a tiny number of us are still running these old machines. We're not economically significant for them. I think they just don't know, and I'm wondering if there's anyone at HP who cares enough to get involved and communicate with MS about it.

[SDH]

... hence the term "unsupported hardware".

Note on page 1 of this thread my image of what Windows Update offered me right after a clean W11 25H2 install. That image was captured via a run of Wushowhide.cab literally within 20 seconds after reconnecting to the internet. No KB5066835 there either, but there was another KB that needed to be hidden to prevent install failure. And, your image does not even show that one now.

So, maybe MS already has squashed the bug.

Wait... what? You don't already get a backup done before every update? 🙂

[vanadu]

>>Wait... what? You don't already get a backup done before every update?

I do now 😱😱😱😱😱😱😱

I've never had an update bork my system before, so I didn't see the need. Up until now I've relied on rolling back fickle updates. Now I guess I'll be more cognizant of the s**tstorm these KBs can cause. 

I guess glass-half-empty people would say all this was a waste of time because the situation resolved itself like I suggested it might in one of my earliest posts. But glass-half-full people like me (most of the time) would say this has been a wake-up call for us stone-age HP Zx00 owners because something like this is likely to happen again. Thanks again for all your help on this SDH!