If you receive error "0x800F0A12" when trying to install Service Pack 1 (SP1) for Windows 7 or Windows Server 2008 R2, it means the SP1 installer can’t access the Windows system partition (sometimes called a volume) of your computer’s hard disk to update files.
Follow the instructions below to get more information about the error. Then, expand the section in this topic that corresponds to the message you receive and follow the instructions.
In the list of results, right-click Command Prompt, and then click Run as administrator.
If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
Type bcdedit, and then press Enter.
Make a note of any error message you receive, and expand the section below that corresponds to the message.
You might have opened Command Prompt without using administrator privileges. Repeat the steps above and be sure to select Run as administrator in step 2.
This error can occur if the system partition isn't being mounted, or made accessible to Windows during startup. To re-enable automatic mounting of all partitions and volumes, do the following:
If your computer is part of a network at an organization or business, contact your system administrator before you proceed to understand why access to the system partition was disabled.
Carefully type mountvol /E and press Enter.
Restart your computer, and try reinstalling SP1.
If mountvol doesn't help, expand the sections below to learn more about other possible causes of this error and suggestions on how to fix the problem.
If a disk management tool from another software manufacturer was used to copy (sometimes called clone) disks or partitions on your computer, the SP1 installer might not be able to identify the correct system files.
Turn off your computer and physically disconnect any external disks or drives that aren’t required for starting Windows.
Turn on your computer, and then try installing SP1 again.
If removing all external devices doesn’t help, the disk management tool that was used might’ve designated a hard disk partition other than the Windows system partition as active. The active partition is where your computer looks for the files it uses to start an operating system.
The Windows system partition needs to be the only active partition in order to install SP1. For more information on identifying active and system partitions, see What are system partitions and boot partitions? For information on how to make the Windows system partition active, see Mark a partition as active.
Some disk management tools from other software manufacturers can make certain types of partitions called extended partitions active. If an extended partition is active, you’ll need to use a disk management tool from another software manufacturer to make changes.
In some advanced disk management or server configurations, you can remove hard disks from a computer without shutting down Windows. This is informally known as hot‑swapping disks or drives.
If a hard disk containing the system partition was swapped out, return the missing disk to the computer and try installing SP1 again.
A SAN connects multiple servers and storage devices on a single network. If you are running Windows from a SAN , contact your system administrator for more information. Or, go to the SAN manufacturer’s support website for information on how to make the system partition accessible.
Any other error message indicates a problem with the BCD store, a Windows system file. Rebuilding the BCD store can help fix the problem.
If you have customized boot configuration settings, such as dual-boot settings, the following steps might cause you to lose those settings.
Remove all floppy disks, CDs, and DVDs from your computer, and then restart your computer using the computer's power button.
Do one of the following:
If your computer has a single operating system installed, press and hold the F8 key as your computer restarts. You need to press F8 before the Windows logo appears. If the Windows logo appears, you need to try again by waiting until the Windows logon prompt appears, and then shutting down and restarting your computer.
If your computer has more than one operating system, use the arrow keys to highlight the operating system you want to repair, and then press and hold F8.
On the Advanced Boot Options screen, use the arrow keys to highlight Repair your computer, and then press Enter. If prompted, select the Windows 7 installation to be repaired, and then click Next.
Select the language and keyboard input method, and then click Next.
Select the user name of an administrator account, type the password, and then click OK.
In the System Recovery Options dialog box, click Command Prompt.
Carefully type Bootrec /rebuildbcd and then press Enter.
Restart the computer, and try reinstalling SP1.
If after trying these suggestions you are still unable to install SP1, you might need to reinstall Windows 7. For more information, see Installing and reinstalling Windows 7.