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.
Just had to thank you for this post. I was trying to clone the original drive in my mid 2010 17″ MacBook Pro to a new SSD drive, and no matter what I tried I kept getting the “missing operating system error”, trying to boot into Windows (booting to the Mac OS X worked fine). It was driving me nuts. I finally decided that rather than trying to get the cloning to work properly, perhaps I could fix the not quite working clone. That’s when my Google search came across a post that referenced this post. I followed the instructions, and it worked!!!! Thank you SO much.