So you’ve installed Windows 7 on your Mac using Boot camp. Then you decide that you need more space on your Windows partition. So you use the Mac’s Disk Utility on the partition tab to shrink the MacOS partition. Then you fire up a live CD of GParted to expand the Windows partition. Now Windows won’t boot and you get the dreaded “Missing Operating System” error. This isn’t too difficult to fix.

If you boot from your Windows CD and try to run the start up repair utility that comes with Windows, it can’t find your partition. I’m not totally sure why this is, but there is an easy fix.
A. Open command prompt from Windows recovery option menu. Type “diskpart” at the command prompt.

B. At the DiskPart prompt, type in “list disk”. You will see the list of disk currently attached to your computer.

C. Now type in “select disk n”, where n is the disk number. In this example, disk 0 is selected.

D. Now that correct disk is selected, type in “list partition” to get a list of all the partitions on that disk.

E. To select the partition you want to set as active, type in “select partition n”, where n is the partition number.

F. Now that you have selected the disk and partition, mark it as active by just typing the word “active” and pressing Enter. Now the partition is set active.

G. Exit from the command prompt and run startup repair.

That’s it! Windows will repair the partition and it will boot.