If you receive error "0x800F0A12" when trying to install Service Pack 1 (SP1) for Windows 7 or Windows Server 2008 R2, it means the installer canít access the system partition of your computerís hard disk to update files.

Usually, this partition is accessible to Windows (although you can't browse it in Windows Explorer). However, in some situations, access to the system partition might be blocked after Windows has started. For example:

The system partition isnít automatically mounted, or made accessible to Windows, during startup.

A hard disk containing the system partition was removed prior to beginning SP1 installation.

Windows is running on a storage area network (SAN), and access to the system partition has been disabled.

A disk management tool from another software manufacturer was used to copy (or clone) the disk or partition on which youíre trying to install SP1.

For a detailed read visit Microsoft.