Why pay Apple $69.99 when you can build your own Lion install drive for the App Store purchase price of $29.99 -- plus the cost of an inexpensive thumb drive. Here's how to create a full install on a drive, not just the recovery disk that we recently posted about.

You'll need a copy of the OS X Lion installer. If you saved a copy when you first installed Lion, great. If not, you'll need to re-download it from the Mac App Store.

To do so, launch the App Store and option-click the Purchases tab. An "Install" button should appear next to Lion. Click it to re-download the installer. You can use this option-click-Purchases trick to re-download any purchase, not just Lion.

Once the 3.74-GB installer finishes downloading, go to your Applications folder to find the installer itself. It is called Install Mac OS X Lion. Right-click (or Control-click) the installer and choose Show Package Contents from the contextual pop-up. A new Finder browser window opens, showing the normally hidden material inside the installer bundle.

Navigate to Contents > SharedSupport. There you'll find a disk image called InstallESD.dmg.

Open a new Finder window with Command-N (File > New Finder Window). Navigate to /Applications/Utilities and launch DiskUtility.

Attach a thumb drive to your Mac that is at least 4GB 8GB in size. (Update: some readers say 4GB isn't enough. As you can see, I used a 16GB drive) Prepare it for use by creating a single HFS+ partition. Select the drive (e.g. SanDisk Ultra) in the left hand column. Drives are listed first with their partitions listed after them, each partition indented slightly.

With this drive selected, click the Partition tab and choose Partition Layout > 1 Partition. Choose Format > Mac OS Extended (Journaled).

Click the Options button at the bottom-right of the partition layout. Select GUID Partition Table.

Click Apply. Disk Utility asks you to confirm. Click Partition. Wait as it unmounts, partitions, and remounts your disk.

Next, select the new partition (Untitled 1 by default). Click the Restore tab.

Click Install next to the source field. Drag InstallESD.dmg into the file-open window and click Open. Drag Untitled 1 from the left column to the destination field. Click Restore and agree to Erase the drive and replace it with the contents of InstallESD.dmg. You may have to authenticate as an administrator.

Wait. It will take some time for the drive to be written. Once it's done, eject it, label it clearly, and put it away for a rainy day.

Meanwhile, go out and spend the $40 you just saved wisely.

This article was originally published on Tuaw.
Daily Update for August 11, 2011