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I know there are a number of steps to make a pendrive bootable by changing its file system and all, but my question is, what is the need to do it anyway when I can simply go to the boot menu in my system and giving the highest priority to USB drive so that it automatically loads the OS from the pendrive at startup?

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    If the pen drive is not "bootable" it won't boot, even if you select it as a boot device.
    – CharlieRB
    Mar 14, 2014 at 17:04
  • But I just loaded a new OS without actually making the pendrive bootable. I just copied the OS into the pendrive and inserted it on the startup, and it worked! why? Mar 14, 2014 at 17:07
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    Don't know enough about your pen drive or what you've done to know definitively, but it may have already been bootable in the first place.
    – CharlieRB
    Mar 14, 2014 at 17:09

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In order for a system to boot, it needs a device with some sort of boot loader. For example, in Linux, grub and LiLo are the two popular ones, while Windows NT and above use NTLDR (NT Loader). These functions tell the system how to load the rest of the operating system, and without some sort of boot code, the code on your flash drive won't run.

When you format the pen drive and make it bootable, you set a switch on the device or partition that explains to the BIOS "HEY! I got boot info for you to read and execute!". The BIOS sees this, if it's that device's turn to boot, then executes the code. Without it, the BIOS would just skip to the next device and ignore your USB drive until the OS asks for it.

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  • No problem! If it solves your question, remember to also 'Mark as Answer' using the green check mark on the left Mar 14, 2014 at 17:24

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