after you installed mint to the second disk and wrote grub to the second disk mbr (hd1) when you booted to the second disk did a grub menu appear?
the other think i can think of is that there was no boot flag on a partition on the second disk (I should check my setup for this).
I am thinking that the grub menu for mint is wrong because when you boot off the second disk it then thinks it is the first disk. so your grub menu should look like this.
title Linux Mint, kernel 2.6.17-10-generic
kernel /boot/vmlinuz-2.6.17-10-generic root=/dev/sda3 ro quiet splash
so i changed (hd1,2) to (hd0,2) and sdb3 to sda3.
BTW i forgot you had sata drives and with the new info my mount command should have been:
sudo mount /dev/sdb3 /tmp
don't give up now!