it may be possible to just run the installer with the usb drive attached, then install onto the usb drive and boot from it then run the transfer utility (which I am assuming does exist) to get your stuff off the internal drive.
Been reading about the migration assistant and it maybe the case that I'd have to copy across manually as the migration assistant may copy across the defunct/undetectable files which I'm looking to leave behind..
it may be possible to just run the installer with the usb drive attached, then install onto the usb drive and boot from it then run the transfer utility (which I am assuming does exist) to get your stuff off the internal drive.