It’s very handy to have a bootable version of OS X on a thumb drive.
I have written how to do this using a program called DiskMaker X, and I have written about how to do it from the command line here, but after doing this a few times I find the most reliable method is the one outlined below. It uses Apple’s built-in disk utility.This works with any version of OS X. Mountain Lion, Yosemite, El Capitan etc. Youâ€™ll need an 8G USB thumbdrive.
1. Purchase or download the OS X Installer via the App store. It will look something like this:
2. Right-click on the installer and select â€œShow Package Contents. â€ Open the ‘SharedSupport’ folder and find the file called â€œInstallESD.dmgâ€.
3. Insert the thumb drive and use disk utility to format it as Mac OS Extended (Journaled) with GUID Partition Table.
4. Use Disk Utility to â€˜Restoreâ€™ the dmg file to a thumb drive. Select the thumndrive in disk utility and then go to the ‘Edit’ Menu of Disk Utility and select ‘Restore.’ It will prompt you for a disk image. Select the ‘InstallESD.img’ file.
This will make an OS X Installation USB drive like you buy from the Apple Store.