Mac won't boot from USB drive: Prohibited sign
It may be that boot-up from the USB flash drive is &quot;prohibited&quot; because your USB flash drive requires adjustment to the &quot;ownership&quot; settings. I learned this the hard way myself. Most of the guides, including Apple's own support doc on how to create a bootable Mac OS installer on USB — as of this date — fail to mention the ownership step.
First and foremost, the USB flash drive must support ownership functionality. Complicating matters, the USB flash drive default may disable boot support. For this reason, you will need to enable it before/after completing the process described by Apple, MacWorld and others using the &quot;createinstallmedia&quot; command in the Terminal window.
The &quot;missing step&quot;, after ensuring the USB flash drive is formatted correctly, is to ignore ownership

Mounting the USB flash drive on the desktop, right click and select &quot;Get Info&quot;
In the info window, expand the Sharing &amp; Permissions: section.
Click the padlock icon to unlock administrator options <img src="https://i.stack.imgur.com/xcwz7.png" alt="padlock
icon" />.
Enter your administrative password.
Deselect the check box ignore ownership
Click the padlock to save the changes.
Eject the drive if you're ready to remove the USB Drive and install

On the machine you wish to install on, you can hold down the Option (ALT) key to select the boot drive, select your installer. If you can't see it, reset the PRAM and try again. If you still can't boot from the USB flash drive, verify that you created the Mac OS installer on the USB flash drive while working in the same (or as close to) current Mac OS as you intend to write to the USB flash drive. Some users have problems creating a successful bootable USB flash drive because they attempt to follow Apple's instructions on older versions of Mac OS X that require Disk Utility, not the Terminal window &quot;createinstallmedia&quot; command, to create a bootable USB flash drive. If all else fails, try a different name-brand flash drive of at least 8 gigabytes or greater in size. (Do keep in mind that there are a lot of counterfeits sold by third parties on Ebay and Amazon so it's equally important to buy your USB flash drive new from a reputable source.)
The most thorough guide I have found on the topic of creating a bootable Mac OS installer on a USB flash drive is here:
https://www.lifewire.com/create-emergency-os-x-boot-device-2260173