The Tool Desk
Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →If a Proxmox virtual machine stopped booting immediately after you changed its disk bus, controller, firmware, machine type, or boot order, treat that change as the leading suspect. First save the current qm config <VMID> output and compare it with the last known-good configuration. Restore the previous boot-critical setting before repairing guest files; rollback tells you whether the problem is a missing guest driver, a firmware/boot-entry issue, or a disk attachment problem.
Windows must have the new storage-controller driver available in the boot-start configuration. Linux may need the controller module added to initramfs and its bootloader configuration regenerated. A UEFI shell generally means firmware cannot find a valid boot entry, so check disk attachment, boot: order=, firmware mode, and efidisk0 before attempting filesystem repairs.
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Start with the Proxmox configuration, not the guest filesystem
A hardware edit can prevent a guest from reaching its own operating system. If the VM fails before guest firmware appears, changing Windows or Linux files cannot help yet. Open the Proxmox task log and verify that the VM’s storage is available and that the expected disk is still attached.
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Capture the current state
- Stop making hardware changes and run
qm config <VMID>. - Save that output outside the VM. Compare it line by line with the last known-good copy.
- Pay particular attention to
bios,machine,scsihw, each disk’s bus and volume,efidisk0, and thebootorder. - Record whether the guest used legacy BIOS or UEFI before the change, and whether a virtual TPM was attached.
The comparison is more useful than guessing from the symptom. A disk can still be present in the hardware list but excluded from the firmware boot list, while a correctly ordered disk can remain inaccessible because the guest lacks its new controller driver.
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Use the symptom to choose the first branch
| What you see | Most useful first check |
|---|---|
| The VM never reaches guest firmware | Read the Proxmox task/error log, check storage availability and disk attachment, then restore the previous VM configuration. |
| UEFI shell or “no boot device” | Check bios, boot: order=..., disk attachment, and efidisk0. |
| Windows blue screen 0x7B or INACCESSIBLE_BOOT_DEVICE | Make the target storage-controller driver available at boot, or inject it offline. |
| Linux kernel panic, “cannot mount root,” or an emergency shell | Boot with the old bus, add the target module to initramfs, and regenerate the bootloader configuration. |
| Blank console but network or QEMU Guest Agent access works | Check display configuration and guest reachability before forcing a power-off. |
Rollback is the fastest diagnostic
Restore the last boot-critical property you changed: the old disk bus, storage controller, firmware mode, machine type, or boot order. Change only that property, boot once, and observe the result. If the guest starts, the rollback identifies a compatibility or boot-driver problem rather than immediate disk corruption.
Do not combine a rollback with a firmware conversion or a filesystem repair. Keep the working configuration until you have copied logs, backed up the VM, and prepared the guest for the intended migration.
Repair a UEFI shell or missing boot device
OVMF (UEFI) and SeaBIOS (legacy BIOS) are different firmware paths. The guest’s partition layout and boot mode must match the selected firmware. Switching a healthy installation from one mode to the other can make it appear unbootable without changing a single file on the OS disk.
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Check the four Proxmox-side conditions
- Confirm the firmware setting in
qm configmatches the mode used when the OS was installed. - Confirm the system disk is attached to the VM and points to the expected storage volume.
- Inspect
boot: order=.... Proxmox marks only devices in that configured list as bootable for guest firmware; an attached disk omitted from the list may never be tried. - For a UEFI guest, verify that
efidisk0still exists and was not removed, replaced, or restored from an incompatible backup.
If the EFI system partition and loader are intact, use the firmware boot menu or recovery environment to launch the loader manually. Recreate the Windows boot entry from recovery tools if necessary; do not reinstall Windows merely because the firmware lost its entry.
Separate firmware changes from controller changes
Changing a disk controller and changing from SeaBIOS to OVMF (or the reverse) are independent operations. Perform one, reboot and verify, then perform the other. Retain the EFI variable disk for UEFI guests and include it in backups. A controller fix will not repair a missing EFI boot entry, and a new boot entry will not supply a missing storage driver.
Windows: prevent or repair INACCESSIBLE_BOOT_DEVICE
Windows shows 0x7B/INACCESSIBLE_BOOT_DEVICE when the kernel cannot reach the system volume through the controller presented at boot. A system disk that worked on IDE, SATA, or one virtual SCSI presentation can fail immediately when moved to VirtIO-SCSI or another controller if its boot-start driver is not enabled.
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Safest method while Windows still boots: stage the controller
- Leave the Windows system disk on its known-good bus.
- Add a small temporary disk attached through the target VirtIO-SCSI or other intended controller.
- Boot Windows and allow Plug and Play to enumerate the controller. In Device Manager, confirm the storage controller appears without an error icon.
- Shut Windows down cleanly. Remove or detach the temporary disk only after the controller has been installed.
- Move the system disk to the target bus and place that disk first in the Proxmox boot order.
- Boot once. If it fails, return to the previous bus rather than making another simultaneous change.
The temporary disk is only a way to make Windows load and configure the controller before the boot volume depends on it. It does not replace a backup, and it does not prove that a different firmware mode or machine type is compatible.
When Windows no longer starts: use WinRE or installation media
- Attach Windows Recovery Environment or matching installation media and boot it from the Proxmox console.
- Open Command Prompt and run
diskpart, thenlist vol, to identify the actual Windows volume and the EFI system partition. Recovery environments often assign different letters than normal Windows. - Identify the correct storage-controller driver package and its
.inffile. Keep the driver matched to the Windows edition and architecture. - Inject it into the offline image with DISM, using the letters discovered in recovery. A typical form is
dism /image:C: /add-driver /driver:D:driverscontroller.inf; replace the example paths with the real Windows and driver paths. - Verify that DISM reports the driver was added, then reboot with the target controller still presented. Do not remove unrelated drivers during recovery.
If the disk is visible in WinRE but Windows still produces 0x7B, recheck the controller selected in Proxmox and whether the injected package contains the boot-critical driver rather than only a user-mode utility. If the disk is not visible at all, solve the Proxmox attachment, storage, or controller problem first.
Repair a lost Windows UEFI entry
When the OS volume and EFI system partition are present but OVMF opens a shell, inspect the EFI partition from recovery and use the Windows boot-repair tools to recreate the boot files and firmware entry. Preserve the existing OS disk, efidisk0, and any TPM state. Recreating a boot entry is a narrower repair than reinstalling or re-partitioning the guest.
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Linux: rebuild initramfs after a bus or controller move
Linux distributions load storage support early, from initramfs, before the root filesystem is mounted. If the installed kernel lacks the target VirtIO-SCSI or other controller module in that image, the kernel can panic with “cannot mount root” or drop into an emergency shell even though the root filesystem itself is healthy.
Boot the installed system using the old presentation
- Restore the previous controller or attach the disk through a bus supported by the installed kernel.
- Boot the installed system, its rescue entry, or a distribution rescue environment with the root filesystem and EFI system partition available.
- Check that the target controller module exists for the running kernel and is included in the initramfs. The module name and package differ by distribution and kernel version.
- Install the distribution’s package or module if it is absent, then rebuild initramfs for the kernels that will boot.
- Regenerate the bootloader configuration, shut down cleanly, switch the disk to the target bus, and set the correct Proxmox boot order.
Distribution-specific rebuilding patterns
On Debian- and Ubuntu-family systems, the normal tooling is commonly update-initramfs, followed by update-grub. For example, an administrator may rebuild all installed kernel images with update-initramfs -u -k all and regenerate GRUB with update-grub after confirming the required module is installed.
On RHEL-family systems, the corresponding workflow commonly uses dracut, such as dracut -f, followed by the distribution’s bootloader configuration procedure. Exact commands depend on the release, kernel layout, root filesystem, encryption, and whether the VM boots through UEFI.
These examples are patterns, not universal commands. Verify the distribution, mounted root and EFI paths, kernel version, and bootloader before running them. If the guest uses disk encryption, RAID, LVM, or a nonstandard root layout, ensure those components remain in the rebuilt initramfs as well.
Keep UEFI state intact
For a Linux UEFI guest, retain the EFI system partition and Proxmox EFI variable disk. A controller change should not be bundled with a conversion between UEFI and legacy BIOS. Once the controller is working, verify that the bootloader still points to the same root filesystem and that the firmware boot entry remains available.
Understand the hardware choices before changing them
| Property | Available choices | What must match |
|---|---|---|
| Firmware | SeaBIOS or OVMF | Match the guest’s existing partition and boot mode; changing it can make a healthy installation appear unbootable. |
| Machine type | i440fx-style or q35-style | Record the old type and check storage-controller compatibility before changing it. |
| Disk bus/controller | IDE, SATA, VirtIO-SCSI and other virtual presentations | IDE/SATA are broadly recognized; paravirtualized VirtIO-SCSI needs its guest driver available during boot. |
| Boot selection | Proxmox boot: order=<device...> |
The intended system disk must be attached and included in the configured order. |
Changing machine type can alter the virtual hardware exposed to the guest even when the disk bus is unchanged. Treat it as a separate migration, record the old value, and test it independently.
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Windows running normally
- Back up the VM and save its complete configuration, including EFI and TPM state where used.
- Record the current firmware, machine type, controller, disk mapping, and boot order.
- Stage the target controller with a temporary disk and confirm it in Device Manager.
- Move only the system disk, set its boot priority, and reboot.
- After a successful boot, make the next independent hardware change, if any, during a later maintenance window.
Windows already offline
- Restore the old bus and confirm the disk is attached.
- If rollback is impossible, boot WinRE, identify volume letters with DiskPart, and inject the correct controller INF with DISM.
- Test the controller change alone. Address firmware or boot-entry problems separately.
Linux running normally
- Confirm the target controller module is present for the installed kernel.
- Rebuild initramfs and regenerate the bootloader configuration while the old bus still works.
- Shut down, change only the disk presentation, set boot order, and boot.
Linux already offline
- Restore the former bus or boot a rescue environment.
- Mount the installed root and, for UEFI, the EFI system partition according to the distribution’s recovery procedure.
- Chroot or use the distribution’s rescue tooling to install the module, rebuild initramfs, and regenerate bootloader configuration.
- Reboot with the target controller and verify root mounting before attempting any firmware conversion.
Backups and prevention
- Take a snapshot or backup before hardware edits, and save the full
qm configoutput separately. - Include
efidisk0and, where present, the virtual TPM state in the VM backup. They carry boot identity and security state rather than disposable metadata. - Keep a recovery ISO available and know how to open the Proxmox console and guest firmware menus.
- Change one boot-critical property per reboot: firmware, machine type, storage controller, or boot order.
- For Windows, pre-stage every new storage controller with a temporary disk before moving the system disk.
- For Linux, verify the new controller module is in initramfs before switching buses.
- Keep the last known-good configuration until the new setup has completed several clean boots.
When the console is misleading
A blank graphical console is not proof that the VM failed to boot. If the guest responds to the network, answers through QEMU Guest Agent, or accepts remote connections, check display hardware and console settings before powering it off. A display change can coincide with a storage edit and obscure a successfully running guest.
Conversely, a responsive firmware screen does not prove that the OS disk is healthy. Firmware may simply be waiting because the disk is absent from boot: order=, the EFI variable disk was lost, or the selected firmware mode does not match the installed loader. Use the configuration comparison to isolate those cases before modifying guest data.
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