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If a Proxmox VM stopped booting immediately after you changed its disk bus, storage controller, firmware, machine type, or boot order, treat that change as the leading suspect. First save the current qm config output and compare it with the last known-good configuration. Then roll back the last boot-critical change, test one change per reboot, and only modify guest files after you have ruled out a missing disk, incorrect boot order, firmware mismatch, or lost EFI state.
Start with the symptom, not a repair command
The correct recovery path depends on where the VM stops. A guest that never reaches firmware has a different problem from Windows showing INACCESSIBLE_BOOT_DEVICE, and both differ from a Linux kernel panic. Use the earliest visible failure as your branch.
| What you see | Most likely class of failure | First action |
|---|---|---|
| Proxmox task fails, the VM will not start, or no guest firmware appears | Host-side configuration, storage availability, or a detached disk | Read the Proxmox task/error log, confirm the storage is online, and verify that the expected disk is still attached. Restore the prior VM configuration before editing guest files. |
| UEFI shell, “no boot device,” or firmware returns to its menu | Missing boot entry, wrong firmware mode, incorrect boot order, detached disk, or missing efidisk0 |
Check bios, boot: order=..., disk attachment, and the EFI variable disk. Do not reinstall the operating system yet. |
Windows blue screen, especially 0x7B or INACCESSIBLE_BOOT_DEVICE |
The Windows kernel cannot access the system disk through the newly presented controller | Put the disk back on the previous bus, or make the required storage driver available before trying the new controller again. |
| Linux “cannot mount root,” kernel panic, or emergency shell | The target controller module is absent from the installed kernel’s initramfs, or the root device identity changed | Restore the previous bus long enough to boot rescue or the installed system, then rebuild initramfs and the bootloader configuration. |
| Network and QEMU Guest Agent work, but the console is blank | A display or console configuration problem rather than a boot failure | Verify that the guest is actually reachable before forcing a power-off or changing storage. |
Preserve the working state before changing anything else
Make a snapshot or backup if the storage and workload allow it, and save the complete VM configuration as a separate file. The configuration is the reference point for every rollback.
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qm config <VMID>on the Proxmox host and save the output with a date and VM identifier. - Compare the current output with the last known-good copy. Pay particular attention to
bios,machine,scsihw, each disk’s bus and file,efidisk0, and thebootorder. - Back up the VM’s EFI variable disk when it has one. In a UEFI guest,
efidisk0stores firmware variables such as boot entries; it is part of the VM’s boot identity, not disposable metadata. - Back up any virtual TPM state used by the guest together with the VM. Losing TPM state can create a separate recovery problem even when the operating-system disk is intact.
- Record the exact hardware edit that preceded the failure and avoid making several additional changes before testing a rollback.
Do not infer that a healthy disk is damaged merely because the guest stopped booting. A disk can remain perfectly readable while firmware or the kernel no longer knows how to reach it.
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Compare and roll back the Proxmox configuration
Proxmox exposes several independent boot-critical choices. A change to one can hide the effect of another, so restore the last known-good value first whenever possible.
| Setting | What it controls | Typical failure after an edit |
|---|---|---|
bios |
SeaBIOS (legacy) or OVMF (UEFI) firmware | A valid installation appears unbootable because its partitioning and boot mode no longer match the firmware. |
machine |
The emulated hardware model, such as an i440fx-style or q35-style machine | Device presentation or controller compatibility changes unexpectedly. |
scsihw and disk bus |
The virtual storage controller and the bus used to present each disk | Windows lacks a boot-start driver, or Linux lacks the controller module in initramfs. |
boot: order=... |
The devices guest firmware is allowed to try, and their priority | A disk is attached but omitted from the firmware’s bootable list. |
efidisk0 |
The persistent OVMF variable store | UEFI boot entries disappear or firmware settings reset after the disk itself is moved or replaced. |
For the first test, revert only the most recent boot-critical edit: put the system disk on its former bus, restore the former firmware mode, return the previous machine type, or restore the old boot order. If the VM boots, you have separated a guest-driver issue from a firmware, attachment, or host-storage issue. Shut down cleanly before trying the next individual change.
Repair a Windows VM after a controller or bus change
Why Windows shows 0x7B
INACCESSIBLE_BOOT_DEVICE (bug check 0x7B) means the Windows kernel cannot find or communicate with the boot disk at the point where it must mount the system volume. After a virtual SCSI or NVMe presentation change, the usual cause is that the required storage driver is not enabled as a boot-start driver. The disk may be present in Proxmox and contain an undamaged Windows installation, yet still be inaccessible to the kernel.
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Safest method: stage the target controller while Windows still boots
If Windows can still start on the old bus, load and verify the new controller before moving the operating-system disk.
- Power off the VM cleanly and record the current configuration.
- Add a small temporary disk using the target VirtIO-SCSI, SCSI, or other controller. Do not move the system disk yet.
- Start Windows and wait for Plug and Play to enumerate the temporary disk. If Windows requests a driver, install the driver package appropriate to that guest version and architecture.
- Open Device Manager and confirm that the target storage controller appears without a warning icon. A disk that is visible is useful evidence, but the controller itself must be functioning.
- Shut Windows down cleanly. Detach the temporary disk if desired, move the system disk to the target bus, and set that disk first in Proxmox’s boot order.
- Boot once and verify both the operating system and all expected data volumes. Keep the previous configuration recorded until the migration has been proven.
This staged approach lets Windows initialize the controller while it still has a working boot path. It is safer than changing the system disk and discovering the missing driver only after the machine is inaccessible.
Offline recovery with WinRE or installation media
When Windows no longer boots, use Windows Recovery Environment (WinRE) or installation media. The letters shown in recovery are often different from the letters used during normal operation, so identify them rather than assuming that the operating-system volume is C:.
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- Boot the VM from the recovery ISO and open Command Prompt.
- Run DiskPart, then use
list volumeand inspect volume sizes, labels, and file systems. Assign temporary letters if necessary. Identify the Windows volume and, for a UEFI installation, the small EFI System Partition. - Locate the correct storage-controller driver package containing one or more
.inffiles. Use a driver that matches the Windows release and CPU architecture; do not remove unrelated drivers during recovery. - Inject the driver into the offline Windows image. A typical form is
dism /Image:D: /Add-Driver /Driver:E:DriversStoragecontroller.inf, whereD:is the verified Windows volume andE:is the media containing the driver. Use/Recurseonly when the specified folder contains the intended driver set. - Review DISM’s result and verify that the driver was added successfully before rebooting. If the command reports that the image path is wrong, return to DiskPart and re-identify the volume letters instead of guessing.
- Reattach the disk through the intended controller, set the correct boot device first, and test. If it still fails, roll back to the former bus and investigate the driver package, firmware mode, and boot order separately.
Do not use offline servicing as a reason to change firmware, machine type, and storage controller at the same time. Each additional change removes a useful diagnostic comparison.
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When the Windows disk is present but UEFI cannot start it
A UEFI shell generally means firmware did not find a valid boot entry or executable at the expected location. First verify that the OS disk is attached, that OVMF is still the intended firmware, that efidisk0 was not removed or replaced, and that the disk is included in boot: order=....
Preserve the existing EFI System Partition and the OVMF variable disk. From WinRE, use DiskPart to identify the Windows and EFI volumes. If the EFI loader files are present but the firmware entry is missing, recreate the Windows boot entry with the recovery tools after confirming the letters; for example, a command of the form bcdboot D:Windows /s S: /f UEFI uses D: for the verified Windows volume and S: for the verified EFI partition. Never run that command with guessed letters, and do not format the EFI partition as a first response.
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Repair a Linux VM after a controller or bus change
Restore a boot path before rebuilding the guest
For a Linux panic, “cannot mount root,” or emergency shell, put the disk back on the previous controller or attach it through a bus that the installed kernel already supports. Boot the installed system if possible; otherwise choose the distribution’s rescue entry or boot a compatible rescue ISO. This temporary rollback gives you a running environment in which to repair the next boot.
Ensure the target controller is in initramfs
The kernel used during early boot does not automatically have every module installed in the root file system. The target VirtIO-SCSI or other controller module must be available to that kernel and included in its initramfs before the disk is moved again.
- Check the running kernel and available modules with the distribution’s normal package and module tools. Confirm that the target controller module can be loaded and that the installed kernel has the required support.
- Install the distribution package that supplies the module when it is missing. Keep the currently working controller available until the rebuild has completed.
- Regenerate initramfs using the distribution’s tooling. Debian- and Ubuntu-family systems commonly use
update-initramfs; RHEL-family systems commonly usedracut. The exact options depend on the distribution, kernel version, encryption layout, and whether the image is generated for one kernel or all installed kernels. - Regenerate the bootloader configuration with the distribution’s normal GRUB tooling. Debian- and Ubuntu-family systems commonly use
update-grub; RHEL-family systems commonly use agrub2-mkconfigworkflow. Verify the output path for the guest’s firmware mode before running a command. - Shut down cleanly, move the disk to the target bus, and correct the Proxmox boot order. Test one reboot. If it fails, return to the previous bus and inspect the initramfs contents and bootloader output rather than repeatedly power-cycling.
Commands vary with distribution, initramfs implementation, filesystem, encryption, and bootloader layout. Treat the examples above as tool families, not universal copy-and-paste commands.
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Keep Linux UEFI state separate from storage migration
If the VM uses OVMF, retain both the EFI System Partition on the OS disk and the Proxmox EFI variable disk. A controller change and a firmware-mode change are separate operations. Do not combine them in one reboot: first prove that the guest boots with its existing UEFI arrangement, then make any firmware change only if the installation genuinely requires it.
Understand the hardware choices before trying again
| Choice | When it fits | Risk during a migration |
|---|---|---|
| SeaBIOS | Legacy BIOS installations with a matching partition and bootloader layout | Switching from UEFI can make a GPT/EFI installation appear to have no boot device. |
| OVMF | UEFI installations with an EFI System Partition and persistent variable storage | Missing or replaced efidisk0, lost boot entries, or an omitted EFI disk can lead to a UEFI shell. |
| i440fx-style machine | Existing guests configured for that emulated hardware model | Changing the model can alter device presentation and should be done only after recording the old configuration. |
| q35-style machine | Guests designed and tested with that newer emulated hardware model | A model switch can expose compatibility issues unrelated to the storage driver. |
| IDE or SATA disk bus | Broad guest recognition and useful fallback paths | Performance and device naming can differ from the intended paravirtualized layout. |
| VirtIO-SCSI | Efficient paravirtualized storage when the guest driver is ready at boot | Windows needs the controller driver enabled before the system disk is moved; Linux needs the module in initramfs. |
| Explicit boot order | Controlling which attached devices firmware tries first | An attached disk that is absent from boot: order=... may not be considered bootable. |
Match the firmware to the installation already on disk. Do not switch SeaBIOS and OVMF merely because a bus migration failed; firmware mode is an independent variable.
UEFI-shell troubleshooting sequence
- Power off the VM and run
qm config <VMID>. - Confirm the OS disk file and controller assignment are still present.
- Confirm the VM’s
biosvalue matches the installation’s existing boot mode. - Confirm
efidisk0exists, points to the expected storage, and was included in the backup. - Confirm
boot: order=...names the intended disk before temporary or data disks. - Enter the guest firmware menu and check whether the Windows Boot Manager or Linux bootloader entry exists.
- If the entry is absent but the EFI files remain, use the guest’s recovery environment to recreate the entry. If the disk or EFI partition is absent, fix the Proxmox attachment first.
- Only after firmware can see and select the loader should you investigate filesystem errors or reinstall bootloader files.
When the console is blank but the guest is alive
A blank Proxmox console does not prove that the operating system failed to boot. If the VM responds to its network services or QEMU Guest Agent, check the display adapter, console type, and guest display settings. Obtain logs or connect through the normal network administration path before forcing a power-off. A display-only problem can be made worse by repeatedly changing boot hardware.
Prevent the next hardware change from becoming an outage
- Take a snapshot or backup appropriate to the workload and save the complete
qm configoutput before editing. - Include
efidisk0and virtual TPM state in the backup whenever the VM uses them. - Change exactly one boot-critical property per reboot: firmware, machine type, storage controller, disk bus, or boot order.
- For Windows, attach a temporary disk on every new storage controller and confirm the controller in Device Manager before moving the system disk.
- For Linux, verify that the target controller module is present in initramfs before changing buses.
- Keep a recovery ISO available and know how to open the Proxmox console and guest firmware menus.
- Leave the last known-good configuration and rollback bus documented until the guest has completed a full boot and application check.
The practical rule is simple: preserve the configuration, restore the last working path, then prepare the guest for one hardware change at a time. Windows needs its storage driver staged or injected; Linux needs the controller in initramfs and a consistent bootloader configuration; UEFI needs an attached disk, a valid boot entry, correct boot order, and intact EFI variable storage.
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