That is a bad assumption. it may be true for some drives, and I know it is not true for others. You should verify at the manufacturer's website to find out what the correct jumper configuration it. Or just tell us your drive model and we can tell you the jumper settings.
Also most new computers are now using "CS" (Cable Select) settings for the IDE interfaces. With CS it is the position on the IDE cable string that determines whether it is master or slave.
Note: some CS cable setups make the end connector the (considered) master and the middle connector the (considered) slave. But other cable configurations have that reversed.
You said you unplugged the 2nd IDE drive, but do you have a 1st IDE drive? perhaps a CD or DVD drive? If not, you can't say that having a SATA drive counts as an IDE drive, it isn't.
If your current IDE drive (assuming you have one) is set for CS, then it most likely will be in the "master" position, and if you set your dad's drive as CS and install it on the unused connector, of that same string, then it will appear as "slave."
My dad's computer went down so he gave me his IDE hard drive to try and get the files off. So I plugged it in as a slave (all jumpers were out so it should be a slave drive). I have my vista OS running off a SATA hard drive. I then booted the computer computer and it said something like "bad boot, insert installation disk and hit enter". I then unplugged the 2nd IDE drive and tried to start up my SATA vista drive by itself and it came up with the same error. Is there an easy fix for this or do i have to try a vista system restore?