most BIOS allow you to specify boot order. Until you can install an operating system on your new SCSI drive, you'll want to make sure that the drive containing your current OS is earlier than the SCSI drive in your boot order.
Every BIOS is different - usaully you can see a message when the computer first powers on instructing you to press some key or another for setup. On my Award BIOS, i have to press the Del key; others use the Esc key, others use function keys. Once you're in the BIOS, look for "Boot order"