Are you booted off the USB? That won't work, they usually have a ram drive overlay. You'll have to boot from another OS and mount the USB to edit the files.
If it still comes up read-only trying searching some solutions for mounting in read/write. You might have to recreate the ISO USB and edit the files before first booting off it. Maybe there is a resize script to use the full disk on first boot that does some funky stuff or something.