Catalina - External Hard Drives repeatedly ejected without cause.

Since you suspect complicity with Catalina why not prepare an external drive with with Mojave and boot with it. See if the problem continues?

I've since downgraded to Mojave, not an issue now.... 100% a Catalina issue. If the drives are part of your daily workflow I would HIGHLY recommend reverting to Mojave. In general so much better IMO.

It is discouraging. :(

Is this how you went back to Mojave?

Do you have a Time Machine backup to Restore to/from?

https://support.apple.com/guide/mac-help/revert-to-a-previous-macos-version-mh15216/mac

I removed all the WD software: WD Discovery

WD Utilities

WD Security

rebooted my system and so far 2 days in the drives haven't disconnected, I did not install the WD software again. my computer recognizes the drives both are USB 3's. keeping my fingers crossed this will resolve the issue, will update if anything changes. to reiterate, each time my computer when to sleep it would unmount external USB 3 WD drives, based on someone sharing I uninstalled all the WD software, didn't download it again. computer recognizes the USB drives perfectly fine, it has woken up at least 40 times without any disconnection for the past 2 days.

I've got this problem too. It happened suddenly, and has been an issue with multiple drives. Being a longstanding Mac user, I've encountered this before. It usually happens with USB 3 drives, usually when connected through a hub. As far as I've been able to figure out, it's to do with macOS being unable to adapt to having a USB 3 drive operating at lower bandwidth, which happens when you connect a drive (or multiple drives) through a hub, or when multiple devices are connected to the same bus. The best solution is a direct connection and to make sure they don't share the same bus (just because you have 4 thunderbolt ports, doesn't mean internally they are each separate. On the Mac Mini 2018, for example, the two on the right are connected to one bus, while the two on the left and the USB 3 ports are on another bus.) Another solution that always seems to work is to put a USB 2 cable joiner between the USB 3 drive and where it is plugged in, effectively, this turns it into a USB 2 drive. It slows the drive down, but it makes it more reliable, which can work well for backup or non-critical data drives where speed is less important. This is also a way of controlling which drives get more bandwith and reserving it for those that need it.

It's important to understand, it's not the drives that are faulty. This is a long standing issue that needs to be fixed. My other suggestion is to give Apple feedback on this through their feedback page or write to them directly, so they are more aware of the issue.

https://www.apple.com/feedback/

This seems to be a very, very long saga: and, like others, I'm shocked that Apple has not addressed it successfully long ago.

My experience is this:

• there are random (panic) shutdowns any time I have either or both of my new LaCie 2big Thunderbolt3 drives connected
• this happens whether they are connected directly to the Mac, or via an OWC Thunderbolt 3 dock
• also true whether I am daisy-chaining to the LG 27" 5K monitor or using separate ports to the Mac
• this has been happening with my Mac mini (which has also, on Apple support's advice) had an SMC re-set (no difference) AND on a brand new MacBook Pro 16. - ie it really doesn't seem to be something to don with your machine folks

For the moment, I have connected the LaCie T3 drives using their USB connections (via the OWC T3 dock). It has run this way for about a week so far, with no recurrence of the kernel panic shutdowns.

Clearly, if I'd wanted a couple of USB drives in this set-up, I'd have done it more cheaply with other options - so I do want to go back to T3 connections as soon as possible. If the expected further update to Catalina 10.15.5 doesn't resolve the issue, I'll be applying to Apple for a refund on the LaCie drives.

I have a 4 drive dock. Mountain is able to keep 3 out of 4 going. I add in a 4th and it all goes down. But, 3 out of 4 is pretty good. (the fourth is an empty drive i use for scratch disk), so i'm happy.

When I plug in a drive to a different port, i'm not getting any errors with Mountain, which is a change, because prior to mountain, it didn't matter if everything was on one port or split up, it still ejected.

Highly recommend.

(Also, this is on Big Sur public beta. So, two things to note: 1: problem still here in Big Sur, and 2: your mileage may vary.)

I have the same story...

In April 2020 I asked my family to buy Lacie d2 Professional 8Tb for my BD. And it became rejecting time to time. I bought a new cable - did not help, then I found this discussion, tried to downgrade to Mojave - it worked perfectly fine! To be honest I believed Apple would fix it soon. Maybe in Big Sur I hoped. So, Big Sur is announced and what I see - my Imac late 2013 is not supported!!! Shame on you Apple, now I am forced to downgrade to Mohave to let my Lacie work. I just can't believe this Catalina issue is still not fixed!!! More than a half year wasted.

I see lots of problem reports with Thunderbolt & Catalina, but might try Keep Drive Spinning...

 http://jon.stovell.info/software/

If the G Tech drive with just Thunderbolt proves to be trustworthy I would strongly suspect the power supply. Have you checked with G Tech to see if there is a firmware update that needs to be applied for Catalina compatability?

Yes, I have confirmed: it's been several days with the G-Drive mounted and being backed up to on the MacBook Pro running Mojave. No shutdowns.

And it’s been several days with the WD Firewire drive connected via Apple Firewire> Thunderbolt adapter to the iMac running Catalina with no shutdowns.  

So the problem clearly is that at least some Thunderbolt drives (in my case the G-Drive Pro Thunderbolt) are not compatible with Catalina. Took a lot of trouble shooting to narrow this down, but pretty convinced at this point.

Having a problem myself, consistent with this thread, that only started occurring after my recent "upgrade" to Catalina. My OWC Mercury Elite Pro Dual hardware raid box, using Thunderbolt 2, has been getting the spontaneous dismounts, typically during heavy disk operations. The most recent was just after booting a Windows 10 Parallels virtual machine that resides on the external volume and subsequently trying to start up a Windows app. I have been using this external drive for probably around 4-5 years, never had a problem. The OWC is not showing drive failures in its status lights. This last time, it was so bad when it disconnected, my Mac Pro (late 2013 trash can) froze up, then rebooted.

Let's be very clear, based on the consistent symptoms I'm seeing in this thread. Apple introduced a regression bug into Catalina, specifically with respect to external volumes, perhaps even specific to Thunderbolt ("2", in my case) i/o volume. I have no personal desire to phone apple support, as they are typically clueless to "real world" usage of their products, and I don't suspect there is a culture of q/a that endeavors to actively root out these issues that aren't part of their core delivered hardware functionality.

As an attempt to resolve, I reset PRAM, then ran diagnostics ("D" on startup), all hardware tested successfully, no errors. So I'm left to suspect that this is in fact a bug specific to Catalina, and I would also have to agree with another post suggesting that apple rushed Catalina out the door, as they frequently tend to do. I thought I'd be safe waiting until the 10.15.2 release, but sadly no, this release may have a critical thunderbolt i/o defect. I wonder if Thunderbolt 3 is affected as well.

Update: I was able to recreate the problem on a Macbook Pro after updating to Catalina 10.15.3 from 10.15.1. Additional observation is that as long as there is IO to the drive, i.e. reading from or writing to the drive, things remain connected. The drives never randomly eject so long as there is activity. I will now wait patiently for an update to hopefully fix the issue.