Originally Posted By: Virtual1
So when you replace the dead hard drive in your computer with a freshly formatted drive.... you just boot off.... what exactly? to begin the reinstallation process? Unless it can boot off the app store wink


If you have any disk that still has a "Recovery HD" partition on it, you can boot from that by holding down either the option key or ⌘R at startup. Once booted into Recovery HD, you have several options available to you, including:
  • Re-install Lion. (It will be downloaded from the internet.)
  • Restore from a Time Machine backup (no internet required)
  • Use Disk Utility to Verify/Repair any volume (or its permissions), to erase volumes, to repartition disks, and presumably (I haven't tested this) do an Apple Software Recovery (fancy name for sector-level clone) from any volume to any other.
  • Launch Safari, to access whatever web-based help-line you require.
  • Enter Terminal, to do low-level tasks. (But you have only a subset of the normal command-line tools at your disposal.)
Even if you don't have an available "Recovery HD", you can boot into any Time Machine backup produced by 10.7.2 or later, to get the same options. In particular, you can restore from that TM backup, without need to access the internet.

If you have neither a full bootable clone (from SuperDuper or CarbonCopyCloner, for example), nor a Time Machine backup, nor any Recovery HD partition, you still aren't completely out of options. You won't be able to recover your data, of course (surely no biggie given that you deemed your data too unimportant to bother backing up), but you can still at least re-install OS X, provided that your machine has "Lion Internet Recovery" in its firmware. (Any machine that shipped with Lion pre-installed came with LIR also pre-installed. Machines that shipped in the few months before Lion was released can install LIR as a firmware patch that Software Update will have told you about.)

As mentioned before, holding down ⌘R at startup makes the machine try to boot from a "Recovery HD" partition. Lion Internet Recovery kicks in if such a partition cannot be found: LIR opens an internet connection to Apple, downloads a copy of "Recovery HD", and boots off that. You will need both internet access and an Apple ID, both of which you can configure right there in the startup screen. Re-installing OS X installs the latest version you're entitled to. (You're entitled to whatever shipped on that particular machine, or to whatever version you purchased from the App Store with the given Apple ID. Plus any updates, of course.)