Well, it is possible your BIOS is trying to do that, but you should be able to fix that. There should NOT be any automatic preference for SATA over IDE drives. Go into BIOS Setup and look for where you specify the Boot Priority Sequence. Make sure it uses your correct IDE older drive as the boot device and try NOT to have any mention of the new SATA drive in there. Many will actually set up the sequence as the optical unit first, the correct HDD second, and NO other devices.
I have seen some BIOS's that do this directly in one place - you set up a list of devices only as long as you want, and just do not add any devices you don't want to use. I also have seen other systems in which you set a priority of something like floppy, optical, and / or hard drive, and on a separate line you set WHICH hard drive(s) in the system you actually mean to use.