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  • Writer's pictureGeorge Napier


Back in December of 2020, I wrote a blog post about a recurring issue that I was having with my iMac Pro involving Kernel Panics and what is referred to on the Interwebs as "Watchdog Panic Attacks." I have struggled with this issue to varying degrees since then. Sometimes it was a minor nuisance. Other times it was an annoyance. Often, it drove me near insane. (For more background, see my Seven Circles blog post.)

So it is with great relief (and a healthy fear of jinxing myself) that I am relieved to say: I think I solved the issue. My system is now stable again. And just in case anyone else googles "Kernel Panics" or "Watchdog Panic Attacks" (WDPAs) in the future, here is the final (🤞) resolution to my flavor of this issue.

Big Sur and Watchdog Panic Attacks

The “watchdog panel attacks” continued with varying levels of frequency. A message thread I started on MOTUNATION contains excellent advice from one of its members on resetting launch services and other settings that helped with the issue. (I’ve also adopted these as a periodic Mac hygiene practices, especially after major installs.) However, these actions had become less and less effective of late.

For different reasons, I decided to update to Big Sur. (I admit that I harbored some vague hope that it might help with this issue.) I went through the processes described in the MOTUNATION thread, made a back-up of my HD, and upgraded to Big Sur.

For the most part, the upgrade was painless. There were new audio drivers to install of course, and a few other updates, but it has been ok so far. However, the WDPAs started up with a vengeance. They happened almost constantly, the record being 12 in one day. I was desperate to solve this issue.

Apple support told me that I didn’t have any old KEXT files on my system that were loading up and causing problems, so it had to be a hardware issue. There were a couple of possibilities.

  • I had previously gone through all of my peripherals and isolated what I thought was a culprit device, but maybe another piece of hardware was the problem.

  • For all of its power, I believe the iMacPro has only two TB3 and two USB3.0 busses supporting its external ports. I have a lot of stuff connected to my system via a couple of quality USB hubs, a Cal Digit TS3+, and a couple of smaller Cal Digit TB3 hubs for multiple external monitors and USB connections. I had been careful about how my hubs/devices were connected to the ports to make sure I didn’t overwhelm one of them, but maybe a problem managing USB traffic was causing these panics.

So I had to start the process of figuring out if either (or both) of these was the case.

Testing USB Connected Hardware

I rationalized all USB connections to two 10 port hubs each on a different USB bus. (I figure the first two physical USB ports feed one bus, and the last two feed another.) I then disconnected one hub at the end of the day and left the computer running overnight. If it rebooted during the night, the problem is still connected to the system and thus is NOT something connected to the hub that was disconnected. (I am making the assumption here that there is only one device causing the problem.)

So, I disconnected hub ‘a’, ran the computer overnight and it had rebooted by morning. (So the problem hardware was still connected.) I reconnected hub ‘a’.

The next night I disconnected hub ‘b’, ran the computer overnight...and it rebooted! So the problem was not something connected to either USB hub. This narrowed it down to the things connected to one of the four TB3 ports.

Testing TB3 Connected Hardware

I won’t go though the gory details, but I essentially did the same process over the next several nights with the devices connected to the TB ports. Turns out that when I removed the CalDigit TS3+ Hub, the WDPAs stopped completely.

That narrowed the issue to the Cal Digit! But that also left the question of which of the devices connected to the CalDigit might be causing the WDPAs. So I went through the same process, but this time only with the devices that were connected to the Cal Digit as the test units.

The Culprit is Found!

Eventually the problem piece was identified: It was the CalDigit unit itself! Whenever that unit is plugged in (even with no other devices plugged into it), a WDPA is sure to occur. Without it plugged in to the computer, the system is rock solid.

I found that to be odd as the Cal Digit is regularly reviewed as the “top” TB3 hub for a Mac. I bought the unit in July of last year and, according to my notes, these WDPAs didn’t start until after a Catalina security update in December. So it worked fine for awhile. Maybe the unit just “went bad” around the time of the update. It’s running the latest firmware, so the utility won’t let me re-flash it AFAIK. (FYI: I’m looking for alternatives if anyone has any recommendations. I’d like a dock that has a downstream TB3 port, at least one USB C port, and a few USB 3.1 or better Type A ports. Anything else is a bonus.)

Anyway, removing the Cal Digit TS3 solved the problem. It was an external hardware issue after all.

Final(?) Thoughts

I hope this info will help someone. If this happens to you, the advice in this MOTUNATION thread is pure gold. Also, ring up Apple Support if you can. If there are issues after that, then a meticulous examination of every connected peripheral is necessary. It’s a tedious and time consuming process, but organizing things so you can examine groups of peripherals at a time, then focusing in on the dodgy piece worked quite well for me and took less time. It’s not a perfect testing regimen - and there are several ways it could have failed - but it got the job done. Maybe it will for you too.

Hopefully, this will be my last post on this subject...

Oh, and A Nice Coda to the Story

I had disconnected my Sonnet external PCI box housing my UAD Octo card months ago because I had identified that as the culprit. Out of curiosity I reconnected it to see if the system would remain stable. It has and for days now!

So as a bonus, I have my UAD plugins back!

Peace everyone.

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