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What are the reasons for/benefits of booting an Operating System from a CD instead of from the hard drive? What kind of uses does it provide that booting from the hard drive does not? Do the answers to these questions vary depending on the OS in question?

Assume the Operating System on the hard drive is capable of booting.

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This has the phrasing of a homework question? – pjc50 Oct 6 '11 at 12:37
It's just curiosity. When I was booting XP from a CD on a machine where it wouldn't boot from the hard drive, I started wondering about the reasons to boot it from CD when it DOES boot from the hard drive. May sound homework-y because I was trying to be clear/specific. – yoozer8 Oct 6 '11 at 13:35
up vote 8 down vote accepted

You may need to boot from a CD for various reasons:

  • Try a new OS. Especially the Linux LiveCDs are a good example. See the Ubuntu LiveCD page
  • Repair your current computer or OS. In this case too, Linux LiveCDs (such as Knoppix) are a good example. Even if this is not your case here. See the Knoppix website
  • Trying to hack a computer, with distros like Ophcrack for instance to find a windows password

Actually, booting from CD is not a regular way to work on the everyday life because:

  • it can be quite slow (this is probably debatable and depends on your computer configuration)
  • you cannot save your context (except if your booting from a USB Key (see wikipedia article on LiveUSB and add some parameters)

You really shouldn't actually work by booting from CD.

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A Live CD session operates entirely in RAM with no hard drives mounted, unless, the user mounts them. Typically, a router can be turned off while the Live CD boots into RAM, and then any changes made prior to turning the router on thus enabling the Network connection.

Contrary to common belief, the Live CD session can Disable the Network connection, mount a hard drive, and any downloaded files can be saved to the hard drive, then the hard drive can be unmounted, and the Network connection re-enabled.

All installed OSes by definition expose the hard drive to the Network because the installed OS needs to reference the hard drive for programs to execute and the like while a Network connection to the Internet is active. This is not true of a Live CD session where the Live CD is read-only, and executes entirely in RAM with no hard drives mounted during the session.

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You can use an Ubuntu LiveCD to surf porn websites without worrying about viruses. Reboot and no viruses! Yay!

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+1: for awesomeness. – surfasb Oct 6 '11 at 17:56
Or you can use it to install Ubuntu and not worry about viruses at all... – user21187 Oct 10 '11 at 0:23

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