Yes, when this happened what do the LEDs show? Also, yes, if it was installed using a UFS filesystem then power outages like that can certainly cause booting issues. However we now install everything as ZFS by default. Steve

@quasaur The issue at hand here is the driver hasn't been written for Apple Silicon by Prolific yet. There are a number of (not-free) apps in the Apple App Store that can supplant the driver need and allow you to do the direct-action in the OS to the device -- I've used a few of these over the years myself. And I haven't found a single USB->Serial device it cannot process yet. You may find recommendations in the forum here from other users for Console applications for macOS that would fit the bill. I know of at least two that have 15- or 30-day free, no subscription trials that you can test run with to see if they fit your needs.

You're workaround also fixed the issue for me! The issue on my machine was not because of manual disconnects; it was when probably renewing DHCP every 24h. Sometimes it would not come back online and it would give: « CHAT: The modem is not responding to "AT" at ModemCmd: label. » Error in log. power_off and power_on solved it without having to reboot the whole machine. Seems like for some reason /dev/cuaU0.2 was locked also but it doesn't appear in the logs.

Thanks again Derelict for your valuable insights I did see the Startech cards when I was searching for a good fit, and I am now ruing that I didn't order one. It looks like I have done my dough $$$ on the card I now have... Your are absolutely correct - I didn't think it would be so hard to get things working. This was supposed to be my cheap and economical to run setup. I have a second machine with a Core i5 2500 (95W) that just chews the gas, but the MB has all the goodies integrated... Looks like it's back to the drawing board lol