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: