4

This question is asked often in Ask Ubuntu, sometimes with few hints about the situation.

Please provide a list of possible reasons to help troubleshoot the problem.

4

Three more reasons not on the original list:

  1. Bad USB flash drive. This could even be a new flash drive which I assume to be not corrupted. Ubuntu will boot after making the Ubuntu live USB on a different USB flash drive. It's worth a try if you have two or more USB flash drives.

  2. Reformat the flash drive and reload the Ubuntu ISO from an application for making a live USB.

  3. Voltage irregularities that affect the smooth amount of voltage delivered from the motherboard to the USB flash drive. Possible causes of this are motherboard, power supply and even irregular house current. A very old computer is more likely than a new computer to have issues with the motherboard or power supply that affect booting to a live USB session. Believe it or not the Ubuntu live USB will sometimes be able to boot if the same computer is moved to another location that has a more controlled source of current.

  4. Some applications for making Ubuntu live USBs seem to work better than others. For example, the built-in Startup Disk Creator application is better than most.

flowchart1
(Click images to enlarge)

flowchart2

  • For #3, a laptop is a good alternate test system; laptop switching power supplies make very good filters, turning whatever wonky voltage coming from your wall into smooth (usually) 19VDC. – Doktor J Nov 22 at 16:43
  • I have also observed that current from laptop adapters is reliable. – karel Nov 22 at 17:03
4

There are many reasons a persistent USB might not boot.

BIOS

  • USB not set as first hard drive in BIOS

  • Problems with BIOS or UEFI boot partitions or files.

  • Secure Boot is not turned off

  • Drive not compatible with computers BIOS or UEFI boot mode

  • Incorrect partition table

  • Out of date BIOS/UEFI firmware

  • Junk in volatile memory

GRUB

  • Incorrect root partition in grub

  • Incorrect path to ISO in grub

  • Incorrect persistent-path, (if used), in grub

  • Grub menu entry structure not suiting OS

  • Incorrect file type for vmlinuz and initrd (.efi and .lz)

  • The word "persistent" is missing from grub.cfg, txt.cfg, syslinux.cfg or text.cfg

Persistence (casper-rw and home-rw)

  • Persistence partition is not an ext filesystem

  • Persistence file not on FAT filesystem

  • Persistence file/partition reused from different version

  • Persistence file full of data, or file update attempted

Hardware

  • Corrupted flash drive, reformat and reload

  • Bad flash drive

  • Not enough RAM to run Ubuntu

  • Bad or incorrect USB socket

  • Incompatible computer CPU

  • Incompatible computer GPU

  • Computer does not meet minimum specs, a lighter version of 'buntu is required

  • Motherboard voltage irregularities

  • Motherboard BIOS limitation with multiple USB devices

Software

  • Bad MD5SUM / corrupt ISO file

  • Modified or corrupted ISO9660 partition

  • USB was removed from computer before ISO file is completely copied

  • Out of date boot drive creation tool

  • User inexperienced with boot procedure

Thanks to karel and sudodus for help with the answer. See Why Doesn't my Bootable USB Boot

3

Great idea to create a list and a flowchart for this subject :-)

I would like to add

Problems with the UEFI/BIOS system of the computer, that it does not recognize a USB boot drive, that works in other computers:

  • the user does not know how to get to a temporary boot menu and how to get into the UEFI/BIOS menu system to modify the settings, should be possible to help when the computer brand name and model are known

  • secure boot might prohibit USB boot in general or a boot drive made without completely signed software, should be possible to turn off secure boot

  • the boot drive is not made for the boot mode of the computer
    • a BIOS mode booter will not boot in UEFI mode
    • an UEFI mode booter will not boot in BIOS mode (alias CSM alias legacy mode)
    • should work with USB drives cloned from Ubuntu iso files that can boot in UEFI as well as in BIOS mode except the mini.iso, which boots only in BIOS mode
  • junk in some volatile memory of the computer or connecting electronics after reboot, should work after shutdown and waiting for a minute.
  • quirk in some HP computers in BIOS mode, that will not boot via USB and grub with GPT, should be possible to fix by using an MSDOS partition table
  • some computers need an update of the UEFI/BIOS system. @oldfred often solves problems in Dell computers: "Dell typically needs UEFI update, SSD firmware update & change from RAID or Intel RST to AHCI for drive". Updating may help also with other computer brands

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