Depending on how you created the USB boot drive, it will be read-write or read-only.
A cloned drive created with the 'Startup Disk Creator' in Ubuntu 16.04 LTS or newer versions, has a hybrid iso9660 file system, which works both in DVDs and USB pendrives (and memory cards), but it is read-only by design, so you cannot edit anything in it. Also 'Disks' alias
gnome-disks, and 'mkusb' (when cloning and creating live-only drives) make USB drives with a hybrid iso9660 file system.
An extracted drive, for example
- a persistent live drive created with mkusb or
- a live-only drive created with Rufus,
can be edited. You can add
nomodeset and other boot options to the boot configuration file, so that they will persist (survive to be used in the future). Edit
boot/grub/grub.cfg in partition #3, 'usbboot' for mkusb. You may need to boot from another drive in order to mount the FAT32 file system read-write for Rufus.
New tips to make your USB boot drive with Ubuntu 19.10+
You can edit your binary iso file and replace the cosmetic boot options
quiet splash with the boot option
'persistent ' or
'nopersistent' (the strings contain 12 characters, so the replacement can be done without any offset in the following part(s) of the iso file. You can do it with
sed 's/quiet splash/persistent /' standard.iso > persistent.iso
sed 's/quiet splash/nopersistent/' standard.iso > nopersistent.iso
and clone the modified iso file, or flash directly with
sed 's/quiet splash/persistent /' standard.iso > /dev/sdx
sed 's/quiet splash/nopersistent/' standard.iso > /dev/sdx
where x is the device letter for the USB pendrive.
You can do it with a safety belt using
It is more convenient and also safer to use
mkusb-plug for the whole process including the
sed commands, and there is a graphical user interface.
You can do it yourself with an extracting method and get whatever combinations of boot options, that you like, according to this link