Windows – Why can’t Windows 7 boot after changing the active partition

bootwindows 7

Situation: Quite some time ago I was running Windows XP, and I installed Windows 7 on a separate partition in case I had to go back. I finally decided to repurpose the old Windows XP partition, and was tired of being asked to select an OS every time my computer booted, so I switched the "Active" partition to the Windows 7 partition. Disk Manager displayed a message along the lines of, "Make sure this partition has an operating system installed, or else your computer won't boot!" Of course, it does have an OS installed, but my computer won't boot – I'm getting a "BOOTMGR is missing" message.

I booted my computer from a Linux disc to switch the system partition back. What I want to know is:

  1. Why was my Windows 7 partition not bootable? Shouldn't the Windows 7 installer have made that partition bootable?
  2. What could I have done ahead of time to detect that this would happen, and to prevent it?
  3. How can I get the Windows 7 partition to boot, so I can wipe the old XP partition entirely?

Best Answer

It's not that the partition isn't bootable (or you wouldn't get this far), it's that the boot loader isn't there and the disk order changed. Windows 7 startup repair can fix this. You can also do this from the recovery console. (I think the command is fixboot or bootrec /fixboot.)

You can't really prevent this ahead of time because you can't reliably predict what the disk layout will look like when the boot device is changed without changing it. The recommend processes is to fix it after it breaks.

If you punch "BOOTMGR is missing" into your favorite search engine, you'll find dozens of articles explaining how to fix this.

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