Based on a method posted on MacRumors here I have been able to create a USB thumb drive which was able to bootable by holding down the option key during OS X El Capitan reboot. There are many sets of instructions online which use dd, but the process below is simpler and doesn’t require use of the command line. The diskutil burn option used by some methods is no longer in the graphical user interface of the disk utility under OS X El Capitan making the solution below preferable.
- Open Applications Folder/Utilities/Disk Utility
- Plug in a blank USB, or at least one you are willing to wipe
- Format USB to Mac Extended (Journaled)
- This will create a default partition on the USB of GUID
- Unmount created Partition (Do not eject it. Use the Unmount button at the top of the screen.)
- Open the disk image from the File menu of the disk utility.
- Select the blank USB on the external list. By default called ‘Untitled’.
- Select Restore.. from the Edit menu of the disk utility.
- Select restore from: the name of your .dmg or .iso image.