Virtual vSAN Witness host upgrade fails with HardwareError

Products

• VMware vSAN
• VMware vSphere ESXi 8.0

Issue/Introduction

Symptoms:

• The CLI upgrade of the Witness host to ESXi version 8.0U3 fails with the following HardwareError:

"Hardware precheck of profile ESXi-8.0U3g-24859861-standard failed with warnings:
<BIOS_FIRMWARE_TYPE WARNING: Legacy boot detected. ESXi servers running legacy BIOS are encouraged to move to UEFI. Please refer to KB 84233 for more details.>"

Environment

• vSphere ESXi 8.x

Cause

• Starting from vSphere 7.0 and all newer releases, VMware does not support adding new server models using legacy BIOS on the VMware Compatibility Guide.
• vSphere 8.0 has officially dropped support for legacy BIOS boot mode and only supports UEFI boot firmware.
• Because the Witness appliance is configured to use BIOS boot, the upgrade fails.

• /var/run/log/hostd.log shows a HardwareError confirming that the host is booting in legacy BIOS mode:

  2025-12-03T06:03:36.879Z In(166) Hostd[132239]: -->    errMsg = (string) [
2025-12-03T06:03:36.879Z In(166) Hostd[132239]: -->       " [HardwareError]",
2025-12-03T06:03:36.879Z In(166) Hostd[132239]: -->       " Hardware precheck of profile ESXi-8.0U3g-24859861-standard failed with warnings: <BIOS_FIRMWARE_TYPE WARNING: Legacy boot detected. ESXi servers running legacy BIOS are encouraged to move to UEFI. Please refer to KB 84233 for more details.>",
2025-12-03T06:03:36.879Z In(166) Hostd[132239]: -->       " ",
2025-12-03T06:03:36.879Z In(166) Hostd[132239]: -->       " Apply --no-hardware-warning option to ignore the warnings and proceed with the transaction.",
2025-12-03T06:03:36.879Z In(166) Hostd[132239]: -->       " Please refer to the log file for more details."

• To verify the boot mode:

  • Edit the vSAN Witness Appliance VM

  • Go to VM Options --> Boot Options

  • Confirm that Firmware is set to BIOS

Reference KB: Deprecation of legacy BIOS support in vSphere.

 

Resolution

Follow the steps below to successfully upgrade the Witness host:

1. Verify that all vSAN objects are healthy before making changes.
   • If the Witness host is already in maintenance mode, objects might show Reduced Availability, which is expected.
2. Confirm that no vSAN resync tasks are running.
3. Place the Witness ESXi host into maintenance mode if it is not already.
4. Shut down the Witness ESXi host.
5. Edit the Virtual Witness Appliance VM.
6. Go to VM Options --> Boot Options.
7. Change Firmware from BIOS to EFI.
8. Power ON the Witness appliance VM.
9. Confirm again that no vSAN resync tasks are running.
10. Proceed to upgrade the vSAN Witness Appliance using CLI.