Bootcamp Windows doesn't start up after 10.7 fresh install upgrade to 10.10 Hardware: Late 2009 27" iMac (it's the first generation 27")
OS: Mac OSX 10.7, Windows 7 64 bit on Bootcamp

Last night, I decided to upgrade my OS to Yosemite, and seeing how old my Mac OS is (haven't touched it since 2013, I have a MacBook Air that I use as my main machine), I decided to do a fresh install.

Everything went fine for Yosemite, but after that, my Windows stopped working. It's still showing up in the System Preferences.app Startup Disk panel, but not when I hold down the ⌥ Option key during system boot up. However, if I try to boot into Windows from the System Preferences.app Startup Disk panel, Windows will refuse to boot and show an error along lines of

Non-system disk
Press any key to reboot...


What can I do to start up as Windows again?
Found the problem. So it seems that it's a relatively common problem with Yosemite messing up the partition scheme, deleting the header for the Windows partition.

This guy called Loner T had been helping people on the forum, and one of the victims of the bug had written up a summary of the steps to take to fix the issue. Do note however, that the fix is highly technical, and requires at least a competent understanding of operating systems and the CLI to safely apply it. So if you're uncomfortable (or have no idea what the instructions are asking for) it's better to find a professional to help). Obviously it goes without saying that you should have a backup of all your data before you attempt this.

Link to the fix:
https://discussions.apple.com/message/27109000#27109000