audio interface connection problem

High Sierra blocks newly installed 3rd party drivers by default.

Install the driver again, reboot and follow this procedure:

https://support.serato.com/hc/en-us/articles/115001883474-macOS-High-Sierra-10-1 3-driver-install-process

I'd look at the system logs and see what shows up when you plug in the interface. Look for errors or any other clues as to what may be going on.

Just in case you're not sure where, click on the Apple icon, then About This Mac, System Report, then Logs... you can also check the details under USB instead of Logs. Hopefully this will get you pointed in the right direction.

OK good. If it's there then I'd follow through the chain. Look at the Audio Midi Setup app and see if it is there and with the correct number of inputs and outputs. If it is, then it should show up fine in Logic under Preferences, Audio, Output Device & Input Device.

That indicates core audio can't communicate with the device as far as I know. Guessing you tried the obvious unplug and plug it in while watching it in Audio Midi Setup... Basically that does a reset. If that doesn't/didn't work, then it sounds like it may be a driver problem. Are you or the manufacturer sure that their latest driver works on your version of Mac OS?

I'd say it could be a hardware problem but you said you already confirmed that isn't the case.

Odd...

This may sound like an odd suggestion but don't connect your interface directly to the iMac, use a good quality USB hub.

It has helped others and is worth a try if you have a USB hub knocking around or can borrow one from a friend or colleague. Once connected up, do a cold boot.

Thats a tough one... sorry to hear that. Definitely not a Logic problem though from what you describe.

Best of luck getting it solved!

I'm glad that it fixed it.

It's ugly because the developers of the driver need to implement this warning into the driver.