the ubiquity installer (used by ubuntu and mint) has a known bug that can cause it to overwrite grub on the system running the usb instead of installing it to the target usb. there is some discussion about it here.
the ubiquity installer (used by ubuntu and mint) has a known bug that can cause it to overwrite grub on the system running the usb instead of installing it to the target usb. there is some discussion about it here.