If it's an IDE drive, and a second drive, you set the jumpers so that it's a slave and connect it to the same cable as the first drive (or a second IDE controller on the mobo if available), and it won't boot.

Now you usually have to change the jumpers on the first drive, depending on the mfr, so that it is jumpered as a master instead of a single drive.

That done, when you power up, the slave won't boot.