70-persistent-net.rules
 

I have 2 test machines which I need to move the HDD between however it won't boot on one of them due to the MAC address stored in

rules.d/70-persistent-net.rules

is there a way to add a second MAC to this file or if I wrote a little bash script to delete it before it's queried at boot at what stage would the script need to be called?

Unless you are booting from the network there is no reason that you cannot boot. Only the network adapter will not be configured correctly. So it is unlikely that this is the source of your trouble.

When the machine boots, a second stanza is included in 70-persistent-net-rules. It will most likely be called eth1 while eth0 remains bound to the hardware address of the other machine.

If you look into /etc/rcS.d you see scripts with the name Snnscriptname with nn a two digit number. Create your script with a lower decimal number than script which start the network and udev. The number must be so high that you are sure that scripts which mount the disk and make it accessible have a lower number.

jlinkels