Is it possible to create bootable USB stick by copying unpacked files?

Question

What if instead of using special soft like Unetbootin or Rufus do the following way. Unpack Iso, then select and copy files to the stick via file manager. Will such stick be bootable? If so then what system can be booted this way (Windows, Linux or any)?

Answer 1

It may be bootable, but it depends.

For UEFI, it will indeed be bootable as long as the ISO file included the "EFI" directory with the correct contents – and as long as the USB stick is formatted as FAT32. (Not NTFS or exFAT. Some firmwares may support those but only FAT is standard.)

You can actually make a Windows USB stick this way, after using DISM to split the oversized Install.wim so that it fits on FAT32. (It may also work for Linux distributions, but I haven't really tried.)

For BIOS (aka "legacy boot") it's harder, as the disk needs to have the correct boot sectors (MBR and VBR), which need to match whatever bootloader is included in the disk. For example, if you're making a Windows 10 USB stick, it'll have the "BOOTMGR" file that's the main bootloader – but BIOS can't find that file on its own, it relies on the disk's own VBR to do that.

All of this can be done relatively easily, though – for Windows you have bootsect to install the MBR and VBR, for Linux you might use extlinux or grub-install. Indeed some Linux distributions like Slax come as simple file archives that you extract into a FAT32 disk and use an included batch script to write the Syslinux MBR/VBR onto it.

Answer 2

While copying the files is 99% of the task, it is not enough for creating a bootable USB - the disk must be correctly formatted and the partition table correctly created.

For example, for creating a Windows 10 bootable USB, it must be first prepared using the built-in diskpart utility as follows:

• Run the Command Prompt (CMD) and enter the diskpart command

• In the diskpart window, type list disk to find your USB device. I assume that your hard disk is number 0 and the USB is number 1.

• Type the following series of commands :

  select disk 1
  clean                        (to format the drive)
  create partition primary
  select partition 1
  active                       (to activate the current partition)
  format fs=ntfs
  exit
  

You may now copy the folders and files inside the DVD or ISO to the USB. This will make your USB drive bootable in Windows 10.