choosing which drive to boot off is bad because depeding on how you installed on the second disk and the bios depends on how the second disk is found.
so normally you boot of the first disk, hd0 and the second is hd1.
but if you boot off the second disk is it hd0 or hd1, and which way does the os see them and which way did the installer see them?
the fix, boot as you have been doing (choose the second drive)
so at boot when grub pops up for mint, edit it and where you see hd0,1 or hd1,1 (or similar) swap the number for the hd.
eg hd1 would become hd0 and vis versa. and boot.
once you have booted into mint then you must edit /boot/grub/menu.lst and make the same change.
Hope this helps.
Now where was i going? Oh yes, crazy!