How to mark a partition INACTIVE in Windows 7

When you install Windows 7 SP 1 on a dual-boot system you may receive Error: 0x800F0A12

This error means that the update can not find the 'active' or system partition where the boot files are stored. This partition is to required to start Windows, so it exists, the system just doesn't know which one it is. explains how to set the partition as active, but what if you have more then one partition flagged? Microsoft says that only one partition per drive can be flagged 'active, so what happens if you flag more then one by mistake? Windows Update has no idea which one is correct.

Here are the steps needed to 'unmark' a partition as active:

  • Open up a command prompt and type DISKPART.
  • Type LIST DISK
  • Type SELECT DISK n (where n is the number of the old Win98 drive)
  • Type SELECT PARTITION n (where n is the number of the active partition you wish to make inactive)
  • Type EXIT to exit DISKPART
  • Type EXIT again to exit the command prompt
  • Reboot

 Thanks to for this information. It may be from 2006, but it still works!


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