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Usually, if I want to make a bootable CD for linux (Ubuntu. Mint, Debian, whatever) I just burn the ISO to CD and then restart the machine that I intend to install linux on with that CD inside it.

But to create a "bootable" USB drive, I need to install this software (or its likes) called UNetbootin and use it to map the ISO to the USB drive.

My question is: why?

What is so special about CD technology that it allows me to just burn the ISO to it and install linux from it, whereas with a USB drive I need to go through this intermediary step? Thanks in advance!

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Because the iso image format is a filesystem for Compact Disk. A basic description from wikipedia: ISO 9660, ISO image. So you can burn an .iso directly on CD/DVD, but it require some extra job to put it on an USB drive.

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Thanks @Federico (+1) - but isn't it all just binary? If all the binary data in an ISO file serializes out to 10101010, then it should be persisted to CD and USB all the same...it still doesn't make sense that the hardware can read 10101010 from a CD and boot an OS installer from it, but not read 10101010 from a USB drive and do the same... –  pnongrata Aug 26 '12 at 22:42
    
On boot the BIOS can read a CD file-system from the CD/DVD drive, but it can't read a CD file-system from your SATA drive or USB drive. –  Federico Aug 26 '12 at 22:48
    
Ahhh, so its a BIOS issue... is that just because CD technology is older than USB? If so can we expect a day-and-age where BIOS can read from solid state? –  pnongrata Aug 26 '12 at 22:56
    
The ISO format is only for CD/DVD. There are many other file-system for HD, SSD, USB drive (ext2, ext3, ext4, reiserfs, hfs, hfs+, ntfs, ...). Linux distribution are distributed as ISO file, ready to be burned on CD/DVD. If they make the distribution with other file-system we can do (for example) dd if=image_file of=the_disk and we are ready to boot. –  Federico Aug 26 '12 at 23:09

There's other ways to run linux off a USB, you could simply install it, for one. Its important to understand what unetbootin does - it sets up a bootloader that mounts and boots an iso file directly (or at least expands out its contents in a read only filesystem to do the same). Very specifically it converts a casper type installer into a liveusb. Its simply a program that automates a very specific set of operations.

You could quite easily create a USB image, and image it over to a usb key to get the same result as burning a cd - ubuntu netbook remix's early versions did this.

The 'specialness' of isos/cds in this case is... you're using a spade to dig a hole. You're swapping the spade head for a hammer head to drive in a post, since you don't want to have a seperate hammer. There's no reason you can't use a hammer (a USB specific image, or proper install) to drive in a post (making a USB install).

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