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Information about the cause and possible resolutions of boot-related Blue Screen of Death (BSOD) with error "<strong>UNMOUNTABLE_BOOT_VOLUME</strong>" and stop code <code>STOP: 0x000000ED</code>.

This error is sometimes misspelled as UNMOUNTABLE_BOOT_DRIVE, but is functionally the same.
<h2>About "UNMOUNTABLE BOOT VOLUME"</h2>
[kbintro]
<h3>Description and Symptoms</h3>
The error messages, alerts, warnings, and symptoms below are tied to this error.
<h4>Symptom 1: BSOD with error UNMOUNTABLE_BOOT_VOLUME</h4>
Upon starting a PC running Windows XP, Windows Vista, or Windows 7, a blue screen of death (BSOD) may be encountered with the error message "UNMOUNTABLE_BOOT_VOLUME" and usually with a stop code of <code>STOP: 0x000000ED</code>, similar to the screen shown below:

The full text of this error is written below:
<pre>A problem has been detected and Windows has been shut down to prevent damage
to your computer.

UNMOUNTABLE_BOOT_VOLUME

If this is the first time you've seen this error screen,
restart your computer. If this screen appears again, follow
these steps:

Check to make sure any new hardware or software is properly installed.
If this is a new installation, ask your hardware or software manufacturer
for any Windows updates you might need.

If problems continue, disable or remove any newly installed hardware 
or software. Disable BIOS memory options such as caching or shadowing.
If you need to use Safe Mode to remove or disable components, restart
your computer, press F8 to select Advanced Startup Options, and then 
select Safe Mode.

Technical Information:

*** STOP: 0x000000ED (0x80F128D0, 0xc000009c, 0x00000000, 0x00000000)</pre>
<h4>Symptom 2: BSOD with error UNMOUNTABLE_BOOT_VOLUME</h4>
A variation of symptom 1, with this possible alternate text is this:

The text of this error:
<pre>A problem has been detected and Windows has been shut down to prevent damage
to your computer.

UNMOUNTABLE_BOOT_VOLUME

If this is the first time you've seen this error screen,
restart your computer. If this screen appears again, follow
these steps:

Check to be sure you have adequate disk space. If a driver is 
identified in the Stop message, disable the driver or check 
with the manufacturer for driver updates. Try changing video 
adapters.

Check with your hardware vendor for any BIOS updates. Disable 
BIOS memory options such as caching or shadowing. 
If you need to use Safe Mode to remove or disable components, restart 
your computer, press F8 to select Advanced Startup Options, and then 
select Safe Mode.

Technical Information:

*** STOP: 0x000000ED (0x80F128D0, 0xc000009c, 0x00000000, 0x00000000)</pre>
<h3>Causes of this Error</h3>
A blue screen of death with the UNMOUNTABLE BOOT VOLUME error is an indication that Windows was unable to mount the boot filesystem ((In this context — as in a few other places in Windows — "boot" actually means the system partition and not the actual boot partition (active partition on startup disk 0), per-se.)) to read/load required dependencies from it.

While Windows, being based off a largely microkernel-esque architecture, can load dependencies and components dynamically, there is a basic subset of dependencies that absolutely must be brought online during the boot procedure or else Windows will fail to boot, even if the kernel has been invoked.

With the latest versions of Windows, and especially after the kernel and driver restructuring that took place during the development of Windows Vista, this list of components has steadily dwindled and now comprises only the most basic (and largely universal) input/output drivers ((The basic input/output drivers are, in general, device-agnostic as much of the hardware they interface with has been designed and built around "standards" — even if not everyone sticks to the RFCs and other formal standards as closely as they should!)) in addition to the controllers for the hard disk driver.

If Windows is unable to identify and interface with the hard disk controller (whether integrated into the motherboard or as a drop-in PCI-E card) or if hard disk controller is brought online successfully but Windows cannot read the volume it is being booted from for a variety of reasons, a BSOD featuring the UNMOUNTABLE BOOT VOLUME error may present.

This error has been known to occur as a result of one or more of the following:
<h4>Cause 1: Failed/Incompatible disk controller drivers</h4>
As explained above, if Windows is unable to identify and interface with the hard disk controller responsible for the drive the system partition resides on, the Windows kernel won't be able to read and load the basic dependencies from disk and this blue screen will be encountered.

A common cause that's often responsible for this scenario is an uninstall or failed update/upgrade of the hard disk controller drivers.

For instance, if the Intel or LSI disk management drivers are removed, corrupted, uninstalled, or incorrectly upgraded and Windows does not already have "native," basic support for the particular hard disk controller in use, it will be unable to communicate with the disk adapter to read the required data resulting in the UNMOUNTABLE BOOT VOLUME blue screen of death.
<h4>Cause 2: Attempt to load Windows from wrong partition</h4>
Another scenario where Windows will be unable to load its core drivers and other basic dependencies would be in a situation where Windows <em>has</em> the required drivers to interface with the hard disk controller, but is instructed to load itself from the wrong partition (i.e. not the partition it is actually installed to). When the bootloader (either NTLDR in the case of Windows XP or BOOTMGR in the case of Windows Vista and Windows 7) begins to load an operating system based on the configuration file ((Configuration is stored in the plaintext file BOOT.ini in the case of NTLDR and the binary (registry hive) \BOOT\BCD in the case of BOOTMGR.)) it passes along to the kernel (in the environment block) information about where it should load the operating system from.

If the configuration store (BOOT.INI/BCD) is incorrectly set or no longer points to the correct partition, the ntoskrnl will be unable to load Windows from the indicated partition and an UNMOUNTABLE BOOT VOLUME BSoD may occur.
<h4>Cause 3: Windows partition resized or moved</h4>
In a similar vein to the situation with an incorrectly-configured BCD or BOOT.INI that directs the Windows kernel to load the operating system from the wrong partition, the UNMOUNTABLE BOOT VOLUME error may also occur if the identifier for the Windows partition is changed but the boot configuration file retains the outdated information.

NTLDR and BOOTMGR both have multiple methods of uniquely identifying (or trying to) a partition, some of which are based off a combination of the disk number and the partition offset (how far from the start of the disk) or partition number (how many partitions precede it either physically in the sequential disk layout or in the partition table in the MBR).

If the Windows partition is moved, a partition is added or removed <em>before</em> the Windows partition on disk, or the order of the disks changes, the BCD or BOOT.INI references to the Windows partition may no longer be correct and the kernel will throw the UNMOUNTABLE BOOT VOLUME blue screen when it attempts to load Windows from the wrong partition.
<h4>Cause 4: Changed or reconfigured SATA AHCI/IDE/RAID mode</h4>
Similar to the first situation discussed above wherein the necessary device drivers for communicating with the Windows partition have been uninstalled or incorrectly replaced, it is possible for the drivers that Windows has been configured to load and use at the very start of the boot procedure are no longer correct — even though the underlying hardware has not changed. Most SATA controllers (the onboard Intel ICH ACH controllers, Marvell, JMicron, nVidia, and others) expose an option in the BIOS for the disk controller to be configured in any one of three modes: IDE, AHCI, or RAID.
<ul>
 	<li>IDE is the most-basic mode and treats the SATA controller in the same way legacy IDE controllers treated ATA drives: the controller itself contains almost no read/write-related logic and simply serves as the most ascetic of bridges, simply proxying data to and from the drives.</li>
 	<li>AHCI was a new standard for communication that first saw use in home PCs for disks connected to SATA controllers on newer operating systems, wherein the hard disk controller contains substantial logic pertaining to the read and write procedure.Instead of having the Windows device driver dictate each move of the read/write heads and disk seeks, controllers in AHCI mode will act as "scribes" of sorts for Windows, taking higher-level (more-abstract) read and write orders, and then using its own logic to process the commands as efficiently as possible (to allow for hardware-level caching, read/write order optimization (known as NCQ) to minimize disk seeks and decrease latency, and other optimizations.</li>
 	<li>Newer SATA controllers feature a RAID mode where a mostly-software implementation of RAID is used for setting up disks in RAID 0 ("stripe") for performance, RAID 1 ("mirror") for data redundancy, or RAID 5/6/1+0 for a combination thereof.Traditionally a separate, fully hardware-based RAID controller would be required for such a configuration, but modern SATA drive controllers split the RAID process between device drivers and on-board microcontrollers for a cost-effective solution.</li>
</ul>
As can be seen, the three different operation modes above differ quite-greatly in terms of features and implementation. Each of the above three requires a completely different driver to be used by Windows, even though the hardware itself (both the physical disk controller and the drive) has not changed.

Most computers allow for the IDE/AHCI/RAID mode to be set from the BIOS and changed "on-the-fly" without requiring a wipe of the drives. If Windows was installed and booted using one mode, but a different mode was configured (due to changes in the BIOS configuration, a BIOS upgrade, a reset of BIOS settings, etc.) the existing driver will no longer suffice.
<h2>Fix: Using EasyRE to correct incorrect boot configuration</h2>
For cases where UNMOUNTABLE BOOT VOLUME is caused by incorrect or outdated information regarding the Windows partition in the boot configuration files (BOOT.INI or the BCD) for NTLDR or BOOTMGR, Easy Recovery Essentials' automatic one-click Windows repair can normally recreate the boot configuration with the correct settings and parameters to allow for Windows to boot correctly.

[easyre error="UNMOUNTABLE_BOOT_VOLUME" bootscreen="xp-vista-7-8" guaranteed="false"]
<h2>Fixing "UNMOUNTABLE BOOT VOLUME BSOD" on Windows</h2>
[setupcd]
<h3>Fix #1: Roll back disk controller drivers with System Restore</h3>
Most driver-related causes of this error can be corrected by rolling back to an earlier system configuration with System Restore.

Windows normally makes an explicit System Restore point each time certain drivers are installed, upgraded, or removed — meaning that in 9 cases out of every 10, if the UNMOUNTABLE_BOOT_VOLUME error was caused by a change in device drivers, a System Restore point should be available to reverse the damage.

[partial name="partial_system_restore"]
<h3>Fix #2: Change IDE/RAID/AHCI mode in the BIOS</h3>
If the UNMOUNTABLE_BOOT_DEVICE error is being caused by a change of the SATA disk controller's mode in the BIOS or firmware, the fix can be as simple as entering the BIOS and toggling the SATA controller's "mode" setting. The exact instructions differ from one manufacturer to the other, but the options will resemble what is shown in the screenshots below.

[caption id="attachment_4412" align="alignnone" width="800"]<a href="https://neosmart.net/blog/wp-content/uploads/sites/5/2013/08/BIOS-IDE-AHCI.png"><img class="wp-image-4412 size-full" src="https://neosmart.net/blog/wp-content/uploads/sites/5/2013/08/BIOS-IDE-AHCI.png" width="800" height="625" /></a> SATA Mode American Megatrends BIOS[/caption]

Attempt to toggle between all three (or more) modes until a combination that results in Windows booting successfully once more is found.
<h2>More Information</h2>
[partial name="partial_moreinfo" versions="xp,vista,7,server2003,server2008"]