Thanks,it makes sense also as multiple devices increase the chances that one device can cause mischief. Cables Thunderbolt are no longer passive strips of wire, they are computers in themselves and can wear out, especially the most expensive Fiber cables (that run long lengths so you can isolate your enclosure from the work area, important for audio work).

the Mac Pro 2013 was a start in the right direction, if only all Macs had 6 thunderbolt port options.

Thanks for the update!

During the 2 hour support call w/Apple we isolated devices in my setup of multiple screens and hard drives, and docks etc.

We didn't solve the issue on the call, but the tech seemed suspicious of my 2nd monitor, and after the call the drive dismount issue happened again literally while I was editing (complete nightmare!). My second monitor also stopped powering on as I scrambled to save projects etc.

So I swapped the HDMI cable (I should at least explain my Mac talks to an OWC Thunderbolt 2 Dock and from that is connected an OWC Thunderbay4 RAID, my Ethernet, External Monitor, and a Sabrent 4 Slot HDD Dock) my HDMI for Monitor 2 is fed into this same Dock/Hub and I swapped the cable for a different cable and everything resolved super stable, no errors, and also running more smoothly than in recent memory.

No more drive dismount issue. No more issues with the second monitor flickering during the computer sleep or flickering during operation. I'm assuming the cable was creating some kind of problem with the Thunderbolt 2 Dock (it's a hub) which was subsequently kicking off all the other connected devices to it, which now that Mac is putting hardly any ports on it's computers the responsibility of I/O is being punted to 3rd party devices like the Thunderbolt Dock...which seems risky for Apple's brand but what do I know?

Anyway, this may not be the answer others are looking for, but it was the answer for me and may help others. 

This has been the "interim solution" for 7 years now. Oh, well. Let me know if this works 100% for you or not.

that's useful to know about the cables, though these cables have been solid for some time. I spoke to Apple and have a further call with their senior support soon, but in the interim their known solution to the issue is to do the workaround mentioned above - 

System Preferences > Energy Saver >

Prevent Computer from sleeping auto (Set to ON)
Put Hard Disks to Sleep when possible (Set to OFF)

I actually just lest these settings on today and sent my computer to sleep manually when I went to get the kid from school and waking the computer 2 hours later resulted in no kicked drives, so so far so good. I'll report back if Apple's senior support has any more insight.

Set sleep for never, then just sleep it when you leave for the day. (I recommend for you to unmount at minimum)

Or try setting sleep for a few hours (4) and see if that is workable.

YOu can try different cables also, sometimes the trigger for the eject is cables.

lol, thanks for trying to help, but that's just not going to work for me. I'll unmount volumes in the evening when I wrap, but honestly I need to be able to walk away from my computer during the day - frequently - and not have my drives fall off.

You can unmount your volume (not sure if you have to unplug also) when walking away. Frustrating, I understand.

I have a pretty sturdy setup and I highly doubt I have loose connections. As for telling the mac not to sleep, I've switched that on for today's work session, but the problem arises when I step away form the computer too/take a break and then come back e.g. 10 mins later and I've got a bunch of disk eject messages (sometimes).

I've never had it happen while I was actively working. I agree it may be an Apple issue and I'm hitting up their support too, but as per usual, having to spend time on forums trouble shooting is stealing even more time from my day...

Does it get better if you set not to sleep?

Have you made sure all connections are as secure as possible and are not ever jiggled?

hi there, I'm having a similar issue recently, 

2020 iMac
OWC Thunderbay 4 on an OWC Thunderbolt 2 Dock talking to the mac via a thunderbolt 3 port (with an adapter)

Primarily running Premier/video editing

The mac wakes from sleep with disk eject warnings for the OWC and the other dock I have with multiple external HDD's (JBOD), I just turned on 'prevent computer from sleeping/keep hard disks on' to see if it improves, but your other point is noted that it could just be an issue with the Thunderbolt Controller and out of your hands.

I did see the OWC just disappear on it's own a few mins ago, but it also came back a moment later and Premier didn't offline all the media, so it was just a blip.

Generally the problem has been happening for 6 to 9 months and I've only had this Mac since last November.