Removing a hard drive problem

I am currently dual booting from a 40gb hard drive and a 640gb hard drive. I have no need for the 40gb hard drive anymore, and went to remove it, but when I do and go to boot I get the message "disk boot failure, insert system disk and press enter" on the boot screen before it identifies what OS it is or anything. If I plug the 40gb back in there is no problem, and I can boot up fine, but I get that same error message every time that I try to remove it. Any idea why? If it matters, the 40gb is running as the IDE Channel 4 slave, and the 640gb is running as the IDE Channel 3 master. It's not an urgent matter, I would just like to get rid of the 40gb for convenience. Any help would be much appreciated, thanks.

Ok, now my 40gb hard drive has failed, and only the 640gb is detected when I boot. I really don't want to have to reinstall Vista and lose all my data, so if someone could please help me out I would be very grateful. Thanks.