If you’re trying to install macOS Mojave on an external drive connected to your USB-C and Thunderbolt 3 equipped Mac and get the dreaded No Entry (circle with a slash) screen, there’s an easy fix.
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Here’s the scenario: You connect a USB drive to your Mac with a USB A to USB-C adapter, or with a USB-C to USB-C cable and start the macOS Mojave installation. When your Mac reboots to continue the installation you see a black screen with a white circle and slash. The installation is dead in its tracks.
Here’s the fix: Connect the drive to a Thunderbolt dock instead. In our tests that consistently fixed the problem and let the macOS Mojave installation complete. It even worked when we switched to a Thunderbolt dock after seeing the No Entry screen. We had to reboot after moving the drive to the Thunderbolt dock, and the installation picked up where it left off.
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