Disk Utility in El Capitan isn't quite as intuitive as the old version. However, what you do is select the out-dented physical disk entry for the Lexar, then click on the Erase icon. This will let you do the primary setup for the flash drive after which it can be partitioned. You should understand the booting from the flash drive using an installed system will be painfully slow. However, if you simply intend to make a bootable installer then it will be fine.
Once you have prepped the flash drive nothing more is needed to make a bootable disk than to install OS X or use it for a bootable installer:
Make your own El Capitan flash drive installer using the El Capitan tool:
You can create a El Capitan flash drive installer via the Terminal. El Capitan has its own built-in installer maker you use via the Terminal:
You will need a freshly partitioned and formatted USB flash drive with at least 8GBs. Leave the name of the flash drive at the system default, "Untitled." Do not change this name. Wait for the process to complete which will take quite some time.
Open the Terminal in the Utilities folder. Copy and paste the following command line in its entirety into the Terminal window.
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath "/Applications/Install OS X El Capitan.app"
Press RETURN, enter admin password (will not echo to the window) then press RETURN again.
You need to have the installer in your Applications folder or change the paths in the above command line.
