first of all, i don't think dual boot a htpc is a good idea, like me, i need to turn on the tv (or turn on the tv very fast) when booting the computer. instead of waiting for 5 second for grub, i like to skip the waiting time and go directly to the booting process.
second, you can set your bios to boot the hard disk which the grub installed. it is because grub can handle uuid, you don't need to worry about sda or sdb.
my bios (asus) will let me choose which type of device boot first, my choice is dvd, harddisk and then usb.
i also have the choice of which hard disk to boot first. so i guess, your bios is doing the same thing. may be you can see an option under "boot" and choice which harddisk to boot.