I just found the answer myself::catvery-happy:
To use USB on a 7200 controller, format the usb drive i.e. in vfat format, then copy the firmware image on it.
When the controller is then in cpboot> prompt, you can load the image from external USB via
cpboot> bootf 1:1 arubaos_72xx_6.2.1.4_40075
I somehow could not find which usb partition referred to the external usb drive. As it seems USB0 is the internal flash and USB1 is the external. Maybe someone can confirm this.
Thanks/Cheers