I just happened to be browsing the unanswered posts section at the right time! Ok, so yes, this is slightly more complicated. When you get to grub, press esc, it'll prompt you, just hit enter and it should drop you into a more text-like grub list. Scroll down to the Mint entry, press e (for edit), scroll down to the "kernel" line, press e again, change the sda7 to sdb7, press enter when done. You might also need to change the "root" line to match it (probably change "(hd0,6)" to "(hd1,6)", which is just grub's personal way of saying sda7 and sdb7). When you're ready, press b for boot, and it should boot you temporarily into Mint (If it doesn't, reboot and try just the kernel line change). Then you can change the file as I outlined before, and best of all, you now know which bits to change to make it permanent. I know I've given quite vague instructions, they're more as a guide than anything else, as your exact configuration might be slightly different, but hopefully you'll manage.
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