Its a very busy time period, but I can try a configuration like yours, but it may be a while before I can get to it.

Sure, it well could be the Sonnet card.   Is there any useful information in the panic logs that may point to one place or another?

Dropbox should not have access to low level disk access, so I would guess is not involved.

If you have the externals as "primary" do you get the kernel panic? (anotherwards, is the panic when there are 4 disks, or when you are using the externals?)

It could be the Sonnet card. I think it uses the Mac's built in driver, though. One thing you can try, is the Lycom card is pretty inexpensive and also uses the Mac's built in drivers. Might be worth a try to eliminate another possible cause.

First, I did the dust-blowing etc as suggested, no change in behavior, still panics in the same scenario.

After some research on my own, I am not convinced it would be a graphics-card related panic.  Those tracebacks have different drivers than the tracebacks in my panics.   My tracebacks have AHCI and related block device drivers, while the graphics card panics have drivers specific to the device (NVDA) or other graphics-oriented driver names.

To provide further evidence, here is some info about more experimentation I did on the issues.  

First, you may recall I previously successfully synced my DropBox account to a degraded 1+0 RAID, i.e. only one slice was active, on the external drives, the mirror of that slice was not online at the time.   

For reasons of my own doing, I had to re-sync DropBox again.   After rebuilding the RAID array again and getting the 1+0 fully functional, I tried to sync DropBox, about 350GB worth of mostly large video files (up to 2G in some cases).

EVERY attempt at syncing now resulted in a kernel panic within 20-30 minutes.

After multiple attempts at this along with several more incidents of split mirrors, caused by the restart issue in my Mac Pro, I decided to attempt the DropBox sync again with only one side of the mirror active again.  This time, it was the internal drives making up the slice.  This time, as the previous time in a similar configuration, the DropBox sync completed fully.

Afterwards I re-added the external drives as the mirror, they synced to the internal drives and all is up and running now.

What does this suggest to me?

Given:

* panics always happened with internal slice and external slice mirrors active 

* Dropbox sync succeeded twice with no panics when only one side of the array was active, i.e. the mirrors were split and I was running a slice unprotected/degraded.

  * those successful syncs occurred once on the internal drives, once on the external drives

* a full restore from Time Machine succeeded with the 1+0 also intact

It appears to me that:

- the panic is related to disk access based on the traceback

- the factors involved in the panic scenario are: RAID 1+0 fully in service, large sync from DropBox, maybe something related to large data transfers over a network. 

- the panic ONLY occurs with the mirrored slices are active and synced

- either Dropbox or SoftRAID or the combination is creating an issue which causes a panic.  I'm not saying it necessarily originates with either product's kexts, but it's a place to start.   Obviously other factors could be the Sonnet Tempo card in play or some conflict between large concurrent writes to the internal drive bus and the external drives via PCIe.

I also noted there was a similar thread on the forums with a mac pro 5,1 kernel panic that SoftRAID said was to be fixed in a 5.8 maintenance release.. I don't have the link handy as this is my third attempt at providing this data, and I only had the link handy the first time around.  I think it would be worth another look by SoftRAID developers.

Thanks.

The panic log indicates a fault in the drivers for the graphics processor (GPU.) Possible causes that have been reported include a failed GPU, overheating due to dust buildup or a failed cooling fan, and perhaps a bug that may be triggered by using more than one GPU in a Mac Pro (especially NVIDIA GT 120's in a 2009 Mac Pro) and/or connecting more than one display.

So try pulling the card and blowing the dust out of your machine as best you can (reverse end of a vacuum cleaner), including blowing out the fans, then reseat the card. See if that helps the kernel panics.

I think these drives are the cause of not showinig up after a warm restart. I believe they have a TLER enabled setting that does not play well with the Mac Pro's. It may be possible to contact WD and get a firmware app that can disable TLER. (would require booting from linux, or Windows in some way)

Another idea is this. Get a SSD drive. Put it in the lower DVD slot, the connector is already there. Then move all 4 drives internally. That will stop the mirror splitting, although you need to eliminate the cause of the kernel panics. It will make your machine much faster also. You do not even need a "fast" SSD, as the SATA in that slot is limited to 300MB/s, but that is much faster than any SSD and APFS does not work well on HDD's anyway.

Enclosure is a MediaSonic 4-bay enclosure model HF2-SU3S2

I am connected via the eSATA ports

I have attached a support file; however, at the time of collection (now) I have rebuilt everything and all is back in order, except I am hoping to get some advice before resuming the DropBox sync again.  I actually need to do some things besides rebuild my disks for a change :)  I wish I had gathered the TS file when things were in their funkiest of states earlier but I just wanted to get it rebuilt at the time.

I did boot in Safe Mode prior to the most recent mess, hoping any kernel cache issues might be cleared up and that could have been the cause of the crash, but obviously that didn't help.

One idea I'm toying with is moving my dropbox sync folder to another drive that is a regular Mac APFS or HFS drive and ensuring it works that way (as it did once before on the active half of the split mirror) then copying the files back into my home folder on the RAID.  Assuming that all worked I'm not sure DropBox would be entirely happy with that once I moved the sync folder though.

What brand PCI card do you have?

What brand is the enclosure?

This info and support file may help me diagnose this.

At this point I'm on the verge of ripping out SoftRAID and finding another solution.   The disk boot problem, well I can throw money at that and find a pair of disks that show up when the mac KPs or restarts or updates software and at least not tempt SoftRAID with trying to figure out the world after such shenanigans.   

I need a solution.  I SoftRAID to consistently keep the external drives as primary no matter if the internals show up or not.  This is regardless of whether I can solve the issue with the disks disappearing on a restart.  I need to be able to sync DropBox on the mirror without a KP.   Maybe next time it splits mirrors I'll do the sync before recreating the mirror part of the 1+0.  I'm running completely out of ideas.  The ONLY other variable is that I did run MacOS update after recovering the first time and it did two security updates, I believe.  

What else can I provide to you folks to get some help here?  I've spent a week as of today trying to get this back up and running solid again.