As of this morning, OSX 10.8 “Mountain Lion” is out. It’s a whopping $19.99 on the App Store and a couple hour download (depending on your internet connection). If you’ve got multiple machines to install, or just like to be prepared, you can make a bootable USB thumb drive of Mountain Lion. It’s really easy, in fact it is the exact same process as making USB drives for 10.7.
Step 1 — Buy Mountain Lion on the App Store and wait for it to download.
Mountain Lion Install Screen
Step 2 — Find an 8 GB USB thumb drive (preferably already erased and formatted to ‘Mac OSX Extended’).
Step 3 — Ignore the install screen when it appears. Open finder, go to your Applications folder, find “Install OS X Mountain Lion“.
Step 4 — Right click (Or Ctrl+Click) the installer, and click “Show Package Contents“.
Show Package Contents
Step 5 — Navigate to Contents > Shared Support. Copy the InstallESD.dmg file to somewhere safe (like say your desktop).
InstallESD.dmg is the important file
Step 6 — Launch Disk Utility. Goto the Restore tab. Source = InstallESD.dmg. Target = Your USB stick. Click Restore, and wait about 20 minutes (depending on your USB drives speed). Note: I received an error: “Could Not Restore — Invalid Argument” upon completion of the USB restore. This does not appear to be a problem as the USB based upgrade still worked fine.
It’s important that you have the correct source & target, eelse you’ll nuke something you care about.
Step 7 — Enjoy!
That’s it. The process is really quite easy and saves you from having to re-download Mountain Lion on each machine. I’d also suggest you keep around the “InstallESD.dmg” file in case you ever need to make another USB drive, as you won’t be able to download Mountain Lion again on any machine you’ve already upgraded. Happy Upgrades!