What MIGHT happen is this: when you install the HDD, your BIOS detects that change and, all by itself, re-sets the boot order to use that unit first. You would not know this without checking. So, install the HDD and then boot up immediately into BIOS Setup. Go to where the boot priority is set up and make sure it is still booting from the SSD, AND that the 2TB HDD is NOT anywhere in the boot sequence. SAVE and EXIT and see if that works.