Fix INACCESSIBLE_BOOT_DEVICE

Boot error INACCESSIBLE_BOOT_DEVICE is caused either by missing driver for bootable storage device, or by wrong state of HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices registry key.

Learn how to fix INACCESSIBLE_BOOT_DEVICE by updating reference to disk C: in SYSTEM registry hive using Emergency Boot Kit.

Contents

1. Symptoms of this problem

When Windows 10 is loading, an error message like this appears:

Your PC ran into a problem and needs to restart.
(If you'd like to know more, you can search online later for this error: INACCESSIBLE_BOOT_DEVICE)

Windows 10: Inaccessible boot device


On previous versions of Windows, equivalent error message occurs on the blue screen of death and contains the following reference:

*** STOP: 0x0000007B

Windows: STOP 0x0000007B

2. Cause of this problem and scope of this article

This error may occur in multiple contexts and due to multiple reasons:

  1. In Windows Setup due to missing/invalid drivers for storage device containing system partition (aka disk C:). In this case, please read our another article Solving BSOD Issues During Windows Installation
  2. As a result of moving Windows OS between storage devices of different types: SATA, RAID, NVMe, USB, SCSI, IDE. (Windows now requires different storage driver to boot.)
  3. As a result of switching between AHCI/IDE mode of SATA disk controller in BIOS setup. (So essentially this a variant of case 2 listed above: Windows OS was moved from IDE to SATA storage device or vice versa, and now requires different storage driver to boot.)
    If you've done that and now getting INACCESSIBLE_BOOT_DEVICE error, then please read these articles: Change IDE to AHCI without reinstalling Windows (that's for Windows 10, 8.1, 8, 7 and Vista) or Change IDE to AHCI without reinstalling Windows XP (that's for Windows XP only).
  4. As a result of CMOS battery degradation on the systems with RAID setup (always happens simultaneously with system clock reset): BIOS setup settings are reset to their defaults, RAID option ROM is no longer loaded at boot time, and mirrored RAID-1 storage devices appear as separate underlying SATA storage devices to the OS. (So this is also a variant of case 2 listed above: Windows OS was moved from RAID to SATA storage device, and now requires different storage driver to boot.)
  5. Out of nothing: you aren't in Windows Setup, your Windows OS is not on broken down RAID array, you didn't change disk controller options in the BIOS setup, and you didn't manually move your Windows OS between storage devices of different types.

The rest of this article assumes case (5) only. For other cases please refer to other articles listed above or to the official Microsoft documentation on Bug Check 0x7B: INACCESSIBLE_BOOT_DEVICE.

3. Synopsis of INACCESSIBLE_BOOT_DEVICE error

Windows maintains a database of mapping between drive letters (such as C:, D:, E:) and volume identifiers (containing disk signature + partition offset for volume on MBR disk, or GUID for volume on GPT disk). It is stored in HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices registry key.

This mapping can become obsolete or invalid if/when partition corresponding to disk C: is moved on disk, if/when neighbour partitions on the same disk are resized or moved, due to interrupted registry transaction, due to power loss during Windows update, due to failed disk block allocated to the \Windows\System32\Config\SYSTEM registry hive and other similar reasons.

The essence of INACCESSIBLE_BOOT_DEVICE error is that Windows is no longer able to figure out where is its disk C:

If driver for underlying storage device is present in Windows OS, then INACCESSIBLE_BOOT_DEVICE error can be trivially fixed by updating reference to disk C: in the HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices registry key.

4. How to fix INACCESSIBLE_BOOT_DEVICE (step by step guide)

1) Download Emergency Boot Kit and deploy it to USB thumbdrive according to the instructions, then set up your BIOS to boot from USB thumbdrive. You need full version of Emergency Boot Kit to actually write changes to the disk, but it's recommended to try demo version to make sure  there are no hardware incompatibilities.

2) Boot the Emergency Boot Kit:

Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Main Menu
Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Main Menu

3) Choose Mount & Boot Center in the main menu, wait while it loads:

Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Starting Mount and Boot Center
Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Starting Mount and Boot Center

4) Make sure Mount & Boot Center looks like below (partition with Windows OS which should be mounted as "C:" isn't actually mounted as "C:"):

Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Initial Screenshot of Mount and Boot Center
Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Initial Screenshot of Mount and Boot Center

5) Set proper mount path for system partition. It's always "C:" for Windows 10/8.1/8.1/7/Vista, and it's usually "C:" but may be different for Windows XP. If unsure, run Emergency Boot Kit Registry Editor and check HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Setup\BootDir:REG_SZ.

Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Final Screenshot of Mount and Boot Center
Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Final Screenshot of Mount and Boot Center

6) Click OK to close Mount & Boot Center (changes will be commited to disk at this point), then choose "Reboot" in main menu of Emergency Boot Kit:

Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Boot Loader is Restored, Rebooting
Using Emergency Boot Kit to fix INACCESSIBLE_BOOT_DEVICE - Boot Loader is Restored, Rebooting