I have two external hard drives.I want to keep one plugged into a usb port permanently (let us call this one A) and another plugged in every now and then (let us call this one B). Both have Linux operating systems on them.
I want to boot from A if B is not plugged in and from B if it is plugged in.
How can I achieve this? For completeness my current bios look like the following:
It currently seems to be booting simply from the one it last booted from. (If it helps A and B will always be plugged into consistent usb ports and the laptop has no internal hard drive).