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

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 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  • Out of date BIOS/UEFI firmware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 menuentry structure not suiting OS

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

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

PERSISTENCE (CASPER-RW & HOME-RW)

  • Persistence partition is not ext file system

  • 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

  • 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, lighter version of 'Buntu required

  • Motherboard voltage irregularities

  • Motherboard BIOS limitation with multiple USB devices

  • Computer does not boot USB's. Plop boot manager required.

SOFTWARE

  • Bad MD5SUM / Corrupt ISO file

  • Modified or corrupted ISO9660 partition

  • USB removed from computer before ISO file is completely copied

  • Out of date Boot drive creation tool

  • User inexperienced with boot procedure

Please let me know of any reasons I have missed.

An up-vote for any currently valid original reason not on the list, A green check for best original list additions

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  • 1
    I'm not sure what this question is asking. It is interesting though... Nov 5 '19 at 16:04
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    @WinEunuuchs2Unix The reason why I added a flowchart to my answer was to give this question a suggestion to add more direction to it. I suppose this flowchart could be expanded and improved, for example by adding UEFI/BIOS and Syslinux groups to it.
    – karel
    Nov 5 '19 at 17:04
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    @karel I was hoping that you would submit one of your flow charts. It might be a big chore putting every reason on it but it sure would be enlightening. Nov 6 '19 at 1:52
  • I’d consider most of this “question” being an answer actually (see What do we do with this question that is really an answer?). I think the complete list should rather be a (community-wiki) answer to a short, generic question than to be spread among the question and multiple answers.
    – Melebius
    Nov 12 '19 at 10:55
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    @C.S.Cameron I think we should just reopen this and then you can move your list from the question into a new answer. The other answers can remain. (Or, honestly, it could even mostly stay as it is. You could ask, "Besides the following, what..." But moving your list into its own answer would probably be better.) Nov 22 '19 at 5:32
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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. Check the Ubuntu ISO that you downloaded against its SHA256 checksum to verify that the checksums match.

  4. 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.

    Here are a few suggestions that worked when I tried them.

    1. Don't plug in USB devices like mouse and keyboard next to each other. Plug in one USB device at the front and one at the back.

    2. Disconnect unnecessary devices when booting the USB to reduce the power load, for example disconnect one HDD if there are two drives. If you have a 4-port USB splitter with switches on each port this is ideal, because you can turn the USB mouse off when you don't need it and you don't need to open the computer case to disconnect an internal drive in order to reduce the power load.

    3. 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 or connected to an uninterruptible power supply (UPS).

  5. Overheating may cause the computer to power off when trying to start the Ubuntu installer. The CPU temperature can be checked by accessing the UEFI/BIOS setup utility when the computer is booting.

  6. 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.

  7. In case the BIOS Boot options do not recognize the USB device at all, some older BIOSs do not even correctly label USB flash drives in the Boot options. Instead they use some non-standard nomenclature for flash drives like USB-FDD or USB-HDD. USB-FDD stands for "floppy disk drive" which never works with USB flash drives, so in this case you should choose USB-HDD instead. Even though a USB flash drive is not a hard disk drive choosing this option frequently works.

If this was part of a flowchart I would group all of the hardware reasons as child nodes branching out from the same parent hardware node. Similarly I would group the Casper nodes together and the grub nodes together, branching out from a Casper parent node and a grub parent node respectively.

flowchart1
(Click images to enlarge)

flowchart2

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  • You added a nice flowchart about possible problems with the UEFI/BIOS system, but the text is so small, that it is difficult to read. I have rather new glasses, but it is difficult anyway, even when I enlarge the picture in fullscreen with 1920x1080 resolution. Would it be possible to re-design it, so you can use larger font in each box?
    – sudodus
    Nov 6 '19 at 6:05
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    Fullscreen won't enlarge it as well as clicking anywhere on the image to show it in full size. Yes, it will be possible to redesign it, and it won't take any extra time to do it because I'm planning on redrawing all the charts eventually anyway. Right now I just want to get some charts posted to illustrate the basic troubleshooting concept, and encourage other users to post more content that will help me to draw the branching and navigation.
    – karel
    Nov 6 '19 at 6:09
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    @karel: Perhaps a heading for software might be ok to cover applications for making Ubuntu live USBs, Corrupt ISO/MD5SUMs, etc. Nov 6 '19 at 12:11
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    I'll do that category.
    – karel
    Nov 6 '19 at 12:15
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    Great improvement of the flowcharts. Now it is easy for my old eyes to read the text :-)
    – sudodus
    Nov 7 '19 at 10:55
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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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  • Good stuff @sudodus : Some good items that are not so obvious to the average user. Nov 6 '19 at 1:55
  • THIS all is precisely why i only use bootable DVDs when installing an OS. It takes a little bit longer to load, but it works generally without problems. Nov 6 '19 at 11:18
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    @BasLamerichs, Yes, booting from DVD can be easier to manage, but many new computers are delivered without any DVD drive.
    – sudodus
    Nov 6 '19 at 11:22
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    @BasLamerichs Not sure any of my DVD writers still work, I think they are getting rusty. Nov 6 '19 at 12:10
  • @sudodus This question was put on hold, please repost your answer at askubuntu.com/questions/1190764/why-doesnt-a-bootable-usb-boot/… Nov 22 '19 at 6:06
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I have one desktop pc which cannot boot from usb drives. (older hardware 2005/2007) So i burned Plop linux [1]: https://download.plop.at/ploplinux/19.4/live/ploplinux-19.4-x86_64.iso on a dvd (which the older system did support) and booted from it. The first thing you get to see is a bootmenu, with the option for installation of a bootmenu (usb bootmenu), its down a few lines. I installed it, and not the whole distro, only that bootmenu ! After reboot i could simply boot from an usb device, by using the USB/CD/HDD bootmenu. (looks quite fancy too!) Mind that installing an other distro from a USB bootable device onto the computer then will remove the USBbootmenu - But you can install from USB with this method !

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    I have used plop lots of times, here in Sri Lanka not everyone has the latest hardware. It is also possible to put plop on a small corner of the hard drive so that the CD is not needed. I will add "Computer does not boot USB's. Plop boot manager required" to the list. Thank you. Jul 12 '20 at 10:26

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