I have just installed 12.10 on an external HD alongside 12.04 but when I boot it defaults to 12.04. When I press f12 to change the boot sequence it defaults to the external HD but only allows me to boot 12.04
On some BIOS you have to change both the boot order and the boot priority in order to get them to work correctly. Boot priority where the actual device is specified will often taken precedence over boot order where you are more like specify the type of device (HDD, Optical, USB etc)
Also, where did you install grub to when you did the installation? If you installed to the external drive it's most likely that it installed the new grub to the external HDD by default. This is OK, you just need to update the grub on the internal drive so that it knows about and can boot off the external device. First boot into 12.04 and make sure the external drive is plugged in, then open a terminal window with CtrlAltt and type in the command line:
This will let grub find the OS on the external drive and add it to it's boot menu. Next time you boot off the internal drive 12.10 should be listed as one of the boot options.