Do you know what drives you have booted from recently? If you check your settings to see what drive you are booting from you know what drive is being accessed first.
Although, should you be able to access your physical boot settings before your operating system boots, you might be able find this information, like I have, it is important not to change anything or save anything unless you have a solution in mind by using those settings.
Those are settings that, at the very least, could keep you from booting your operating system. F2 or F12 (both labelled hopefully) - one may allow you to access settings - have been my only hope of looking into my computer's operation before I even start an operating system.