If you are a user of a; Clarett Thunderbolt, Saffire or Red interface, on macOS 10.13+ please see the following article in addition to the below steps:
On rare occasions, a Mac preference file relating to audio devices may become corrupted. This can cause a device to not show up as an input/output device in System Preferences > Sound even when there is nothing wrong with the device itself, or cause the device's MIDI ports to not show up. On these occasions, deleting a few preference files can sometimes fix the issue.
The first set of files to be deleted can be found by navigating to Mac HD > Library > Preferences > Audio. There should be two preference files in this folder:
Any additional files in the Audio folder will likely be duplicates (which can often be the cause of the problem), you should delete these files as well.
The other preference file that can cause a similar problem is found in a different Library folder.
On OSX 10.7 and later, open Finder > click 'Go' at the top of the screen > Hold down the "Alt/Option" key and click on the Library folder > Preferences > delete 'com.apple.audioMIDIsetup.plist'
On OSX 10.6 and earlier, the User Library folder is not hidden and so can be accessed by navigating to Mac HD > [your username] > Library > Preferences > delete 'com.apple.audioMIDIsetup.plist'
Note: This filename is lengthened to 'com.apple.audio.audioMIDIsetup.plist' on systems running OSX 10.10 Yosemite.
There are 3 Library folders on a Mac so if you cannot see one of these files it’s worth double-checking that you are navigating to the correct one. After trashing these files restart your computer.
Please note: these files will be re-generated by the operating system after you restart, this is perfectly normal.
If the issue persists, contact Technical Support