Windows 7 SP1 installation failed with error 0x800f0a12

You might encounter an issue during the installation of SP1 for Win7/R2 where the installer exits with: 0x800f0a12. Usually it means the installer can’t access the system partition of your computer’s hard disk to update files.

This is due to the automount policy for your machine being set to disable. To resolve this, use the following steps:

1. Run DISKPART

2. automount enable

3. Restart

4. Install SP1

If the system partition isn't being mounted during startup, you can use "mountvol /E" in Command Prompt (Need administrator priviledge) to re-enable automatic mounting of all volumes.

And ensure that the System Reserved partition is marked active (you can do this in Disk Management or DISKPART).

Also you can try:

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.

Reference:

http://social.technet.microsoft.com/Forums/en-US/w7itproinstall/thread/f955a5ff-3838-4e02-9d44-e698e1ae0346/

http://social.technet.microsoft.com/Forums/en-US/w7itproSP/thread/965ec430-bddf-4a0d-9cfe-a80500fb6fcf/

http://social.technet.microsoft.com/Forums/en-US/w7itproinstall/thread/69ddc500-84e5-4b57-baa3-d51c5249f840/

http://ctp.social.technet.microsoft.com/Forums/kk-KZ/w7itproSP/thread/d7e1de83-2285-4a29-abcf-a4fcf5f471d4

http://ctp.social.technet.microsoft.com/Forums/kk-KZ/w7itproSP/thread/3a82e162-a5de-4702-a632-11305603b223

http://int.social.technet.microsoft.com/Forums/en/w7itproSP/thread/9083e953-f06c-4041-8977-115235359e23

http://windows.microsoft.com/en-US/windows7/windows-7-windows-server-2008-r2-service-pack-1-sp1-installation-error-0x800F0A12

http://blogs.technet.com/b/joscon/archive/2011/02/17/windows-7-2008-r2-service-pack-1-fails-with-0x800f0a12.aspx