Created a bootable El Capitan installer using Terminal and the command "createmediainstaller" per Apple support:
Create a bootable installer for OS X - Apple Support
My iMac will not start from that USB stick. The progress bar advances to what appears to be completion, then the machine hangs while showing a circle with a slash through it.
What's up with that?
iMac (Retina 5K, 27-inch, Late 2015), OS X El Capitan (10.11.5)
That circle with a slash means that something went wrong during startup. In order to discard that the OS X El Capitan installer you downloaded was damaged, try redownloading it and creating the bootable USB drive again to see if the issue goes away. Alternatively, you can try an user-friendly app like DiskMaker X to create the USB drive.
Apart from that, it is also possible that you're trying to start up from an unsupported OS X version for your iMac (that may happen if you've not created the USB drive with the latest OS X El Capitan installer).
iMac (Retina 5K, 27-inch, Late 2015) won't start from USB
