UNMOUNTABLE_BOOT_VOLUME 0x000000ED – Windows cannot mount the C: drive

Fix it now

0x000000ED means the I/O subsystem tried to mount the boot volume and failed. Parameter 2 is the status code the file system returned explaining why, and Microsoft’s published order of work is Startup Repair, then chkdsk from the recovery environment, then the boot records, then suspect the drive.

diskpart
list volume
exit
chkdsk X: /f /r

1. Start the recovery environment. After two consecutive failed starts Windows brings it up on its own; otherwise boot installation or recovery media.
2. Try Troubleshoot, Advanced options, Startup Repair first. Microsoft lists it before anything you type.
3. If that does not help, open the Command Prompt and run diskpart, list volume, exit. The Windows volume is often not C inside recovery, and running chkdsk against the wrong letter wastes an hour.
4. Run chkdsk X: /f /r against the Windows volume. The /r pass reads every sector and takes hours on a large disk.
5. If it still will not mount, repair the boot records with bootrec /scanos and bootrec /rebuildbcd, or rebuild them with bcdboot X:\Windows /s S: /f ALL using the letters diskpart reported.

Image the disk before running a repair pass if you can. chkdsk /r writes to a device that has already failed to serve a mount request.

If the volume mounts and Windows starts, stop here. The next section covers reading parameter 2 and the codes that are not this fault at all.

Why it happens

Mounting a volume is the file system driver looking at what is on the disk, recognising it, and building the in-memory structures that let the rest of the system use it. Everything else waits on that: the registry, the drivers, the session manager. If the boot volume will not mount there is no point continuing, so the I/O subsystem stops with 0x000000ED.

Parameter 1 is the device object of the boot volume and parameter 2 is the status code the file system returned to explain why the mount failed. That second value is the only thing on the record that says what actually went wrong, and it is the part most walkthroughs skip. Read it from the BugCheck event in the System log if the machine ever starts, or from the dump.

Microsoft’s resolution has an order and it is worth following. Startup Repair first, from Troubleshoot then Advanced Options, using bootable recovery media if the machine cannot get there itself. Then CHKDSK /r from the recovery environment command prompt to validate and repair the file system. Then bootrec to fix the master and boot records. And only if none of that works, the possibility that the drive has failed – at which point the vendor’s own diagnostic tools are the next step.

Two codes commonly filed with this one are different faults entirely. 0x00000058 is not a generic volume manager error: it is issued if the system is booted from the wrong copy of a mirrored partition, because the hives say the mirror is valid when it is not and should be pointing at the shadow partition. Microsoft says it is almost always caused by the primary partition being revived, and the documented resolution is to reboot from the shadow partition. And 0x0000008B, an MBR checksum mismatch between what the loader passed in and what the system calculated, is one Microsoft says typically indicates a virus, with a published resolution of booting from write-protected media carrying a scanner and cleaning the infection.

Full reference

The codes, and what each really says

0xC0000098, read properly

This status turns up on boot screens with a file name above it, and it has a published meaning in both of the places it appears. On the boot screen Windows shows ‘File: <binary>, Status: 0xc0000098, Info: Windows failed to load because a critical system driver is missing or corrupt’, and Microsoft’s documented cause is that the binary is from a different version of Windows than the operating system on the machine. The remedy is at file level: check the volume, then replace that specific binary from the component store with a matching copy. As an NTSTATUS value it is STATUS_FILE_INVALID, meaning the volume for a file has been externally altered so the opened file is no longer valid. Neither reading is a broken boot configuration store, so rebuilding the BCD is aimed at the wrong thing.

Recovery environment commands, with the letters that matter

Run bootrec /fixmbr only on a machine that boots in legacy BIOS mode with an MBR disk. On a UEFI machine the boot files live on an EFI system partition and bcdboot is the tool that writes them.

Where cloning goes wrong

A clone that will not mount is usually a clone whose partition table and file system disagree – the tool copied the partition boundaries but not the file system’s own record of how large the volume is, or the target is smaller than the source and something was silently truncated. It is worth checking whether the tool adjusted both, because a volume the file system believes is larger than the partition it lives in will not mount. Where a move between machines is planned rather than a rescue, preparing the source with sysprep /generalize before capture avoids most of this.

When none of it works

• Confirm the drive is detected in firmware setup at all, and consistently across several cold starts.
• Run the drive vendor’s own diagnostic tool – Microsoft’s own last step for this code is that the drive may have failed.
• Attach the disk to another machine as a secondary drive and see whether it mounts there. If it does, the fault is on the original machine’s path rather than in the volume.
• Image the disk before anything destructive, and prefer recovering the data over recovering the installation if the drive is failing.
• If parameter 2 pointed at a device status rather than a file system one, stop repairing the volume and start on the hardware.

Every code this article covers

Confirm the fix worked

1. The machine starts to the desktop on three consecutive cold starts.
2. chkdsk C: /scan from within Windows reports no problems.
3. Disk Management shows the volume healthy and at the size you expect.
4. The drive’s own health tool reports no growth in reallocated or pending sectors.
5. If the code was 0x0000008B, a current scanner reports the master boot record clean before you rebuild anything.

Questions people ask about this