Since OS X Lion is only available via download from the App store, you may be wondering what to do if you need to recover the whole OS. In times past, you’d simply insert the original CD/DVD that the OS came on, but how do you do this if the OS is download only?
Apple has provided a couple of different ways of effecting a full recovery of Lion. In most cases, so long as you are connected to the internet either by Wifi or ethernet cable, you can simply restart your machine while holding down the Command key and the ‘R’ key on the keyboard (Cmd-R).
This will put your machine in recovery mode, and you can follow the instructions from the start up screen. Initially, your machine will try to recover from a special Recovery partition that Lion created on your hard disc when you originally installed it. However, if your HD is corrupted or unavailable for some reason, Lion will attempt to recover the OS directly from Apple’s servers. In cases, be aware that you need an internet connection.
If Lion cannot re-install on your internal disc, or if you want to move the OS to a new or external disc, you will need to go through a few preparation steps. To learn about these and more about Lion’s recovery options, see the full Apple support article here.