So today is the day Apple went ‘back to Mac’ and released the latest version of OS X, called Lion.  Apple have included some 250 new features this time round so there’s plenty of new toys to play with, but something that tweaked my curiosity was the delivery method – Lion is currently only available through the App Store.  You click the link, download the three and half gig installer and away you go.  Seeing as my background is in desktop deployment I’ve been interested to see quite how Apple approach the challenge.

The reason this is potentially quite tricky is that if you’re running an OS, as you are when you download and install Lion, you generally can’t then overwrite that same OS to upgrade it.  At a basic level that’s why you need to reboot OS X and Windows when you do patches and updates, they can’t update running files.  The way around that limitation is to reboot the computer into a second OS which can then be used to update the first.

This isn’t particularly new, it how the previous OS upgrades have worked – the OS boots from the CD – and Windows has been using the bespoke WinPE OS to deploy itself for years.  What’s new with Lion is that there’s no CD or USB disk to boot from.  Instead the installer carves off 650MB of your hard disk and creates itself a new partition from which it can boot.  This new partition isn’t visible within Finder, but the diskutil command line utility is able to see (and mount) it.

When you install Lion, it’s this partition which is booted after the system restart, it then uses the installer file you downloaded to install the shiny new OS. If you look within the new ‘Recovery HD’ partition you’ll find a compressed boot image containing the installation environment and basic system utilities – including Safari.

Once Lion is installed this recovery environment can be accessed either by holding down the option key during startup, where ‘Recovery HD’ is listed as an option, or by holding CMD+R (for recovery).  This will then boot the new environment and give you options to reinstall OS X, recover from a Time Machine backup, change a password or even launch Safari to get online help.  This is good news as it means you can reinstall Lion if you have a problem without having to go back to Snow Leopard and then download Lion again.  Well in most circumstances anyway… I’ve not tried it but I assume it would try to use the original downloaded media.  What happens if that’s not there I’m not sure.

So there you have it.  That’s how it works, which I’m quite pleased about as it’s pretty much what I was doing 10 years ago with Windows 2000 and a fantastic little tool called Partition Magic Virtual Boot Disks.  Those were the days :)

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