I think the one with Primary Partition will boot first.

I think if you press F12 at boot up you can request the drive to boot up.   I think it may retain you choice for the next boot up. 

Pesonally if I was you, I would remove the drive before renting out my house and take with you.   So you chould quickly move the cable to the drive you want to leave.

 

 

Both drives have primary partitions.  One drive is the internal SATA drive.  One will be an external bootable backup copy of the first drive created by Future System's  Casper XP.  I rent my house occaisionally and the renter uses the computer.  Before I leave I erase personal data from the internal drive after copying the drive to the external one.  When I return I need to boot from the external drive and copy it to the internal drive.  For this reason, I need to be able to choose the boot drive.

I do remove the external drive, leaving only the internal one.  If I press F12 will I get sthe choice of which SATA drive I want.  The reason I'm asking rather than testing is that I have not purchased the external drive yet.  I could get a USB drive which I know I can select in BIOS, but it is much slower than a SATA drive.

You could just create a user account (non-Admin priviledges, of course) for the renter and leave your internal HD the way it is. That way, you should't have to choose from which physical drive to boot from.