and thirdly

Create a bootable DVD for OS X Lion

So everyone knows you can grab the upgrade for OS X directly from the Mac App Store. But what if you want a copy on your DVD/USB drive that you can easily boot into for a clean install?

It is still entirely possible.

Here’s what you need.

1. Lion Installer from the Mac App Store
2. Single Layer 4.7GB DVD
3. A Mac

Here’s how to do it.

1. Buy the Lion upgrade from the Mac App Store and wait for it to complete its download. Do not launch it.
2. Start terminal and enter the following

   cd /Applications/Install\ Mac\ OS\ X\ Lion.app/Contents/SharedSupport
   open InstallESD.dmg
   cd /Volumes/Mac\ OS\ X\ Install\ ESD/
   hdiutil convert BaseSystem.dmg -format UDRW -o ~/Desktop/Lion.dmg
   hdiutil resize -sectors 8636968 ~/Desktop/Lion.dmg
   hdiutil attach ~/Desktop/Lion.dmg
   

3. Right now you would have 2 disks mounted in Finder, “Mac OS X Install ESD” and “Mac OS X Base System”.
4. Head into “Mac OS X Base System” and then the “System” directory. Enter the “Installation” directory and deleted the file with an arrow named “Packages”.
5. Copy the “Packages” directory in the “Mac OS X Install ESD” disk into the same directory where you deleted the “Packages” file previously.
6. This copy should take a huge amount of time. Its about 2GB in size.
7. Eject both drives once the copy is done.
8. Switch back to terminal and enter the following

   hdiutil convert ~/Desktop/Lion.dmg -format UDZO -imagekey zlib-level=9 -o ~/Desktop/Lion\ Install.dmg
   

9. Wait for it to do its thing. Once its done, “Lion Install.dmg” on your desktop is a disk image you can either burn to a DVD or restore to a USB drive for a way to boot into the Lion install process to get a clean install.
10. By clean install, it means using the bootable DVD/USB and then running disk utility to format your system to install Lion.

For those interested about the details, here’s what’s going on.

The installer package from Apple actually contains the Packages that you traditionally select as well as the actually OS itself (BaseSystem.dmg). However, due to the nature of its distribution and other really complicated reasons best explained by the people on insanelymac.com, they are separate.

The whole process is basically to grab the OS itself, and squeeze in the packages its supposed to install in order to get a bootable removable disk (BaseSystem.dmg is already bootable).

This makes use of hdiutil to convert BaseSystem.dmg to a writable filesystem and then change its size by resizing it by sectors. The values entered would create a 4.4GB image which is still burnable onto a single layer DVD.

From there on its just copying the Packages into the directory where its symbolically linked.

To wrap it up, use hdiutil to convert the writable image back to a read only disk image with some zlib compression thrown in to reduce the DMG size.