Problem

On macOS 10.13 (High Sierra) or newer, one of the following issues with your audio interface occurs, even though the latest driver has been installed:

• After starting the DAW (Cubase, Nuendo, or another audio application), audio can no longer be played back in iTunes.
• The audio interface selected in the DAW cannot play back or record audio.
• Audio interface cannot be selected in the DAW and is missing in Audio MIDI Setup.

This is what the Audio MIDI Setup looks like when the audio interface is working properly:

In case the audio interface is not listed in Audio MIDI Setup, the driver installation failed and the interface cannot be selected in the DAW.

If Audio MIDI Setup is listing the audio interface as shown on the following two screenshots, the driver installation failed and the device is not working properly.

Error: Audio MIDI Setup showing the UR22C with a black icon.

Error: Audio MIDI Setup showing two entries for the UR22C,
one with a black icon and one with a red icon.  

Background

With macOS High Sierra, Apple introduced a security feature named "User-Approved Kernel Extension Loading" which requires user approval before loading a new third-party kernel extension. This affects audio interface drivers that make use of kernel extensions.

If user approval is missing for some reason, the driver will not be loaded and the corresponding audio interface will not work properly or will not be available, neither in any DAW nor in Audio MIDI Setup.

Kernel extensions must be approved during the first installation of the corresponding driver. For example, when initially installing the Yamaha Steinberg USB, Thunderbolt or Firewire driver or any 'TOOLS for...' software package including a driver, the following warning messages appear:

Warning message: System Extension Blocked
Please do not click the preselected [OK] button!
Instead, click [Open Security Preferences] and follow the instructions under "Solution" below.

Warning message: Yamaha Steinberg * Driver Blocked.
Click [Open "Security & Privacy" Preference] and follow the instructions under "Solution" below.
 

Solution

Go to System Preferences... (Apple menu), open the preference pane "Security & Privacy" and make sure the "General" tab is selected.

If there is a message informing on blocked system software from the developer of your audio interface, click on the pad lock at the bottom left first. Then click on the [Allow] button to approve the kernel extension resp. driver.

Please note that the approval option is only present in the Security & Privacy preferences pane for 30 minutes after the alert. Until you approve the kernel extension, future load attempts will cause the approval option to reappear.

In case the user approval option cannot be brought back, try if it can be made available again by restarting the system. If not, please reinstall the affected driver properly.

Please also reinstall the driver if you experience the difficulties described above, even though the driver approvement seems to have succeeded.

The following instructions on how to reinstall the driver refer to Yamaha Steinberg hardware.

• Disconnect your audio interface by unplugging its cable.
• Download the latest software for your audio interface:
  https://www.steinberg.net/en/support/downloads.html
• Run the uninstaller included in any download for Yamaha Steinberg hardware for macOS (for example, "Uninstall Yamaha Steinberg USB Driver.app").
• The uninstallation requires a restart.
• After the uninstallation has been completed, reinstall the Yamaha Steinberg driver or the "TOOLS for..." application.
• Make sure to follow the instructions to allow the "System software from developer "Yamaha Corporation"" to be loaded in the System Preferences .
• Only after the installation has been completed, connect the audio interface again.