Ubuntu – How to create a bootable live usb that uses ext4 opposed to iso-9660, squashfs and casper

If you run Startup Disk Creator, you'll notice that it does three (four) things:

1. Format the drive
2. Copy over the data from the ISO to the drive
3. (Add an extra casper¹ file for persistence, if selected. dd does not give you persistence.)
4. Install a bootloader

It is not a GUI for dd (there are GUIs for dd, but SDC is not one of those). You can think of it as doing equivalent of Arch Linux's or Gentoo's manual methods. You should be able to create a bootable USB using most (reasonably popular) distros' installation ISOs at present. This was not true about three or four years ago, when I looked at Arch. It was only 2010 that all Arch ISOs gained this feature (where you could write it directly to a USB drive) - from 2008 to 2010 they had special USB images.

Further, if you dd a drive directly, that drive is effectively read-only until you format it or use the remaining space somehow. A SCD-created disk is still usable as a USB drive, even though the contents are are not accessible from the live environment easily. So if you dd a 1GB ISO to a 8 or 16 GB USB drive, you're effectively abandoning the remaining space unless extra steps are taken.

¹Typically, most live images use a SquashFS file for the root partition. SquashFS is read-only, so for persistence, a casper-rw file is created in /casper. I am not sure of the origins of casper. From what I understand, when booted with the boot=casper option, the kernel overlays the casper file (creating one if none exists) on top of the squashfs, so that it gains write capability. casper doesn't have to be a file, it can also be a partition.

As mentioned in the previous comment, this is related to the bug https://bugs.launchpad.net/ubuntu/+source/casper/+bug/1489855 "the sequence of mounting changed from root partition then persistent partition (15.04) to 1st persistent then root partition (16.04) for whatever reason."

There is a work around recently updated in the thread for the bug, which I will elaborate on below as the other thread doesn't seem to have as much visibility (comes up lower in the google search when I was trying to work it out).

1. Create partitions casper-rw and OS

2. Use universal USB installer, etc to set up the OS partition with casper-rw file large enough to be able to make changes - needed for later step to workaround bug (I did it with 2gb, but surely it requires less)

3. Boot up with the new USB. Open /usr/share/initramfs-tools/scripts/casper with root to edit (sudo). Change the function setup_unionfs() to the following : https://launchpadlibrarian.net/258626969/casper%20function.txt

4. sudo update-initramfs -u (Had to uninstall cryptsetup to do this)

5. Copy the generated initrd.img file from /boot to another location.

6. From another OS, delete the casper-rw file in the USB. Copy the initrd.img back to the live partition and change menu item to use the newly built initrd.img.

The next time you boot up from USB, it should now bootup from the casper-rw partition.

This worked for me for 16.04 after a long frustrating time searching for the solution!