With older versions of Ubuntu (8.x at least) the bootloader was never installed on the same hard drive as Windows, if I chose to install Ubuntu to a separate hard drive. I normally have Windows on one hard drive and Linux on another. Note: Not different partitions, but different hard drives! And whenever I needed to boot into Linux I would just press F12 on POST and choose the hard drive with Linux on. That's it! But now, with latest Ubuntu versions it seems to overwrite the existing Windows bootloader, even if Ubuntu is installed on another hard drive? Why is that? And can this be changed when you install Ubuntu?
The hard drive with Ubuntu on is failing, and it is pretty much paper weight as of now. Sometimes it is not identified properly during POST and I get to the Grub rescue prompt. I get the "no such device" error because it awaits the failed hard drive. Because of this I was unable to boot into Windows. You can see the issue here.