You could try booting with the vista drive connected, and when you get to the grub screen delete the "quite splash" from the boot options of the default Linux mint line. When your dropped to busybox you will see a little more information on what it's not finding. Please post back with the error message because It might be looking for something like sdf1.. which then you could reboot and edit the same line again, changing out sda1 for sdf1.. but let us know.
To avoid conflict at startup time with BusyBox - If/when you have a PC with two HDDs, (i.e. one with Windoze, one with Linux), make certain sure that Mint (Linux) is 1st in the Boot queue - Either by editing the BIOS, or as I did, by physically swopping the data leads to the motherboard, and 'tis sorted! Dead easy once you know how!
Incidentally, Vista ain't too bothered where it is in the boot queue, although it does notice that it has been 'demoted' to 2nd ...
Phew! - Ciao 4 now, Chris.