Will chkdsk erase my files? It has been running for hours.

No. chkdsk /f /r repairs the file system and can occasionally move damaged file fragments to FOUND.000 folders, but it does not wipe the drive. Long runtimes are normal because /r scans every sector, so let it finish rather than interrupting it. The real risk is running it on a physically failing disk: image the drive first if the data is irreplaceable.

← Windows boot error index

Fix UNMOUNTABLE_BOOT_VOLUME on Windows 10/11 (Exact Commands)

The fast answer. UNMOUNTABLE_BOOT_VOLUME (stop code 0x000000ED) means Windows found the boot disk but couldn't mount the volume — the file system on it is damaged, the drive is failing, or a bad update broke the boot path. The classic fix is chkdsk from Windows Recovery (WinRE). Open Advanced options → Command Prompt and run it against the Windows volume:

chkdsk C: /f /r

Drive letters can differ inside WinRE — confirm which letter holds Windows first (Step 3 below). If the error appeared right after a Windows Update, uninstall the latest quality update first (Step 2) — Microsoft confirmed in January 2026 that a security update left some Windows 11 PCs unbootable with exactly this stop code.

Why this happens

File-system corruption — a power loss, forced power-off, or unclean shutdown damaged the NTFS structures on the boot volume, so Windows can't mount it. The most common cause.
Failing disk or bad connection — bad sectors on a dying HDD/SSD, or a loose or defective SATA cable, stop the volume from being read. Microsoft's own guidance for this stop code points first at the boot storage device.
Recent Windows Update — Microsoft confirmed that the January 13, 2026 security update (KB5074109) left some Windows 11 machines failing to boot with UNMOUNTABLE_BOOT_VOLUME, especially ones with an incompletely rolled-back December update.
Damaged boot records or BCD — the master boot record or boot configuration data was corrupted, often by an unclean shutdown during a write.
BIOS storage settings — historically, forcing a faster UDMA transfer mode in BIOS triggered this code; a changed storage-controller mode can still do it after a CMOS reset.

Step-by-step fix

Step 1 — Get into Windows Recovery (WinRE) and run Startup Repair

Force three failed boots (power on, then hold the power button to kill it as the Windows logo appears, three times) and Windows opens Automatic Repair. Or boot from a Windows 10/11 install USB and choose Repair your computer. Go to Troubleshoot → Advanced options and run Startup Repair once — it's Microsoft's first-line fix for this stop code and sometimes resolves it outright. If it reports it couldn't repair the PC, continue below.

Step 2 — If it started after an update, uninstall the update

In Advanced options, choose Uninstall Updates → Uninstall latest quality update, then reboot and test. This reverses update-triggered cases without touching your data — it's also the path Microsoft pointed affected users to during the January 2026 KB5074109 boot failures.

Step 3 — Confirm the drive letter, then run chkdsk

In Advanced options → Command Prompt, first confirm which letter holds the Windows volume — inside WinRE it is often not C::

diskpart
list volume
exit

Find the volume with the Windows installation, substitute its letter for C:, and repair the file system:

chkdsk C: /f /r

This is the core fix for UNMOUNTABLE_BOOT_VOLUME. It can take hours on a large or damaged drive — let it finish. Reboot and test.

Step 4 — Repair the boot records

If chkdsk completes but the stop code remains, rebuild the boot records from the same Command Prompt:

bootrec /scanos
bootrec /rebuildbcd
bootrec /fixmbr
bootrec /fixboot

When bootrec /rebuildbcd finds a Windows installation, type Y to add it. Reboot and test.

UEFI/GPT note: on modern UEFI systems bootrec /fixboot can return "Access is denied" — that's normal and not the real fix on those machines. See the bcdboot procedure instead.

Step 5 — Repair system files offline

Still failing? Run SFC against the offline Windows installation (same drive-letter caveat as Step 3):

sfc /scannow /offbootdir=C:\ /offwindir=C:\Windows

Reboot and test again.

Step 6 — Test the drive hardware

If chkdsk reported a long list of bad sectors, keeps finding new errors on every run, or the stop code returns after a clean repair, treat this as a failing drive. On a desktop, power off and reseat (or replace) the SATA data cable — loose or defective cabling is a documented cause of this code. Then run the drive vendor's own diagnostic tool for a verdict. Once Windows boots, check health from PowerShell:

Get-PhysicalDisk

Anything other than Healthy in the HealthStatus column means back up and replace the drive. No command fixes failing hardware — don't reinstall Windows onto a bad disk.

Data safety: If the files on this machine matter and you don't have a backup, image the drive before you run repairs. UNMOUNTABLE_BOOT_VOLUME is frequently the first symptom of a dying disk, and chkdsk /r stresses a marginal drive hard — it can be the push that takes weak sectors from readable to gone. Take a full sector image to a known-good external disk first, and never run repairs on the only copy of irreplaceable data.

GRAM tip: These repairs run in Windows Recovery, which is already on the broken PC. Once the machine boots again, plug in GRAM — a free portable repair toolkit that runs from a USB inside Windows — to check the drive's SMART health, read the crash history, and let its AI agent confirm whether it was corruption, a bad update, or a dying disk, so the stop code doesn't come back on a drive that's quietly failing.

Download GRAM free See AI pricing →

FAQ

Can I fix this without a second working computer?

Yes. You only need to reach Windows Recovery (WinRE), which lives on the broken PC itself — force three failed boots to trigger Automatic Repair, or boot from any Windows install USB. No second computer is required for the repair itself. (GRAM runs inside Windows rather than from boot media — use it once the machine boots to verify disk health and confirm the root cause.)

Will chkdsk erase my files? It's been running for hours.

No. chkdsk /f /r repairs the file system and can occasionally move damaged file fragments to FOUND.000 folders, but it does not wipe the drive. Long runtimes are normal — /r scans every sector, which takes hours on large or damaged drives — so let it finish rather than interrupting it. The real risk is running it on a physically failing disk: image the drive first if the data is irreplaceable.

It started right after the January 2026 Windows update — what do I do?

Microsoft confirmed that some devices failed to boot with UNMOUNTABLE_BOOT_VOLUME after installing the January 13, 2026 security update (KB5074109), mostly machines left in a bad servicing state by an incompletely rolled-back December update. From WinRE, choose Advanced options → Uninstall Updates → Uninstall latest quality update, reboot, and test before trying anything else. If that doesn't get you booting, continue with the chkdsk and bootrec steps above.

Does UNMOUNTABLE_BOOT_VOLUME mean my hard drive is dead?

Not necessarily — simple file-system corruption from a power loss causes most cases, and chkdsk fixes those for good. But the stop code is also a classic first symptom of a failing drive. The tell is recurrence: if chkdsk keeps finding new errors, the error returns after a clean repair, or the vendor diagnostic flags the drive, back up and replace it instead of repairing again.