I feel the answer is on the page here: https://github.com/Hexxeh/rpi-update#root_path-and-boot_path
but I don't understand it to be honest...
I've tried the below in terminal:
sudo ROOT_PATH=/dev/disk5s2 BOOT_PATH=/dev/disk5s1 /Volumes/Storage/Users/Dave/Downloads/rpi-update
but get
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
!!! /dev/disk5s2/root doesn't exist - creating
*** Performing self-update
stat: illegal option -- c
usage: stat [-FlLnqrsx] [-f format] [-t timefmt] [file ...]
this means nothing to me!