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It seems like you can boot from a USB device without a bootloader to me. Even my 6 years old computers that still have a BIOS.

From what I read the BIOS simply looks for an mbr, loads it and executes it. But I have never knowingly written an mbr to any of my USB devices. So how is it that I can still boot from them when certain files are present? I doubt that the mbr could be inside of a file, because how would you ensure that it is exactly at the beginning of the partition.

So what am I missing?

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  • The device is bootable because it was made bootable at some point. It could have been done by you as part of another process. It's impossible to answer without knowing what the devices were, what was on them when you got them, and what you did to them and how you did it.
    – fixer1234
    Jun 24, 2016 at 0:08
  • I usually format my USB devices before copying files onto it for booting. So unless Windows itself is interfering in the process, there should not be an mbr or anything on it. The devices were standard USB thubdrives. Most of them are USB 2, some are USB 3.
    – Forivin
    Jun 24, 2016 at 7:45
  • Who are the people voting to close this question?! It's a damn good question! @Forivin Formatting does not touch the MBR. Use the diskpart command-line tool in Windows to erase MBR + partition information using the clean command. TIP: remove drive letter/mount points first. sevenforums.com/tutorials/…
    – misha256
    Jun 28, 2016 at 3:11
  • @misha256, the question isn't how to accomplish it, the question is how did it get accomplished when the OP doesn't remember doing it. There are all kinds of ways it could have ended up bootable, and the OP hasn't provided any information that would allow readers to do more than speculate.
    – fixer1234
    Jun 28, 2016 at 5:07
  • @misha256 I don't currently have a USB device handy that I could try this on. But as soon as I tried it, I will come back and report if it still booted or not.
    – Forivin
    Jun 28, 2016 at 7:12

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