How do you tell the system which SATA drive to boot to even if both have bootable sectors?
Trying to do boot to my main to access the old but it does not seem to be booting my main.
The BIOS decides the boot order. In many cases you can see all optical drives and hard drives in the BIOS. These too have an order. Place your hard drives in the order you want.
The first partition the BIOS finds having the bootable flag gets selected for booting.
You can enter the BIOS by pressing 'delete', 'f2', 'f12' or 'f8' (Or some other f-key) right after the computer starts and shows the boot screen.