Issues creating Bootable USB for EL Capitan Public beta
I tried many things talked about here and on other top Google searches:


Redownloaded (three times)
Tried moving the .app to Desktop
Changed permissions of .app
Verified the .dmg with hdiutil
Used different Mac


Eventually got it to work by using a different USB flash drive. My original USB was a 16 GB Kingston, the second I tried that also didn't work was a 8 GB Lexar. The third and final one that did not get the "mount of outer dmg failed" error is a PNY 8 GB.

For reference I used the command:

sudo /Applications/Install\ OS\ X\ El\ Capitan\ Public\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan\ Public\ Beta.app --nointeraction