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Can all USB flash drives be used as boot devices if they get the right pre-treatment? Or are there some drives whose hardware makes them not possible to boot from?

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(Question originally said "USB pen drive" but has been edited by @JakeGould to say "USB flash drive". This edit "moves the goalposts" a touch but I'll try my best to provide one update to accommodate it)

Are all USB flash drives capable of being booted from?

Pedantic answer: No. Not all things that physically look like a USB flash drives will be capable of being booted by all hardware. Whether something that physically looks like a USB flash drive can be booted will depend on:

  1. The hardware that is trying to boot it.
  2. The configuration of the hardware trying to boot it.
  3. How the USB device appears to the hardware trying to boot from it.
  4. How the USB device has been set up to be "bootable".

For example, once upon a time there used to be U3 USB flash devices that would appear like a CDROM and then after the software on the "CD part" was installed they would appear like a regular hard disk. As such, just because something is shaped like a USB flash drive doesn't mean it has to present itself to your hardware as a USB hard disk.

Can all USB flash drives be used as boot devices if they get the right pre-treatment? Or are there some drives whose hardware makes them not possible to boot from?

Pedantic answer: No to the first question and yes to the second.

  • If the hardware doesn't know how to boot from USB you won't be able to boot from it.
  • If the USB device doesn't appear like a hard disk/CDROM/floppy disk/some other USB device your hardware knows how to boot then you won't be able to boot from it.
  • If the flash device appears as an unbootable hard disk/CDROM/floppy and you can't overwrite the boot sector/partition table/starting part of the disk then you won't be able to pre-treat it to make it bootable (e.g. read-only device, partially protected device, device that is broken and throws away newly written data etc).
  • And so on.

Interpreted humane answer: When dealing with typical USB flash drives that immediately appear to the hardware as fully writeable hard disks the answer is usually yes - the correct pre-treatment can make the device bootable to modern hardware. That pre-treatment may be specific for different types of hardware (some modern hardware only wants to boot from disks that have EFI partitions on them etc) but you get the general idea.

† The pre-treatment could involve re-programming the USB device firmware entirely so technically I should have said "Yes". Well done - you got me.

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It depends on your hardware platform. Some vendors provide boot firmware that knows how to boot from USB mass storage devices such as USB flash drives, and other vendors might not do so good of a job.

Apple Macs have been able to boot from USB flash drives since almost the very beginning of having USB ports. I seem to recall that the original iMac (the Bondi blue 13" CRT unit from 1998), which was the first Mac to have built-in USB, was not able to boot from USB mass storage at first, but it was added soon after. So basically all Macs built in the last 18 years can boot from USB flash drives. Intel-based Macs (basically all Macs introduced since 2006) require the flash drive to use a GUID Partition Table (GPT) instead of Master Boot Record (MBR), in order to be bootable.

Since we're on the subject, please note that booting from a drive does a lot of small random reads and writes. Some USB flash drives are optimized for large sequential reads and writes, not small/random, and totally suck at small/random. The performance reviews for flash drives usually only list the large/sequential performance, not the small/random performance. So the USB flash drive you thought was super speedy may be a complete dog when you try to boot from it. (Lexar JumpDrive P20 USB3.0, I'm looking at you.)

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Yes, all USB flash drives can be made bootable. However, its the computers BIOS the provides the ability to boot from them.

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  • With this approach you can say one can boot from a card of paper -- if only BIOS provides the ability to communicate with a scanner and chainload the result. If the question was "can I transport any animal by train?" this answer would be like "yes, however the train must be large enough". I tend to read questions verbatim as they are too, so no downvote from me; but the answer is hardly useful. Oct 4 '17 at 8:42
  • @KamilMaciorowski if BIOS supports booting from paper, then why not
    – Keltari
    Oct 5 '17 at 23:39

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