If you prevent the Mac from sleeping when display is off, then obviously you are not going to experience the bug with macOS randomly ejecting external drives when the Mac (not the display) is asleep. Which is what’s happening. But it happened to me already with Sierra, High Sierra and Mojave on a 2013 Mac Pro with Thunderbolt 2 drives, so it’s not new in Catalina. That said, it’s definitely not fixed in Catalina. I had it happen on a new Mac with Thunderbolt 3 drives. Like you, I now prevent the Mac from sleeping when display is off. I guess Apple only cares about saving power and energy (and helping the environment) on paper, and they don’t really care about fixing the bugs that prevent us from putting our devices to sleep in the first place.
Well that didn’t work. This weekend I’ll be back to Mojave.
yeah - Catalina installed on my 27"iMac fine but messed up iTunes so bad it took engineers and senior advisors and myself over 5 months to fix it - they gave me a 200 consolation prize in the apple store for my trouble - down grading to Mojave - learning that 30K Apple Music songs count against your rigid 100K item limit in your library when no one at apple would admit it - my issue now is - I have my newer phone - home pod - iMac - Apple TV and AirPods with apple care+ - I recently called about my airport extreme/networking/icloud music library not working happily with my new home pod - the advisor gruffly told me that if I was not upgraded to Catalina he could only offer me 15 minutes of support advice and then would have to end the call even if the issue was not resolved - pissed me off enough that I fixed everything with a new router without their help - sad that its come to this when a new OS thats been out for 6 months or so is required to get apple care support on new purchases???
I’d love it if you could post a report on how this goes in the ExtraBIT comment thread for this piece:
Hmm… or maybe I’ll break it out into a new thread. Will have to see. 
Will do, but I hope the journey I outline is very boring.
I have had only one issue with my MacBook Pro 16", which of course shipped with Catalina. That’s the oft-mentioned “wake from sleep” issue when connected to my Samsung external monitor (via DisplayPort via an OWC Thunderbolt 3 dock). Sometimes it just won’t wake up. Seems fine when not connected to the monitor. Other than that, Catalina (with everything from my old MacBook Pro via Migration Assistant) has been smooth sailing for me. I do miss TexEdit Plus and Classic Menu (32 bit, not updating to 64 bit).
There’s one improvement with Catalina, though. For many years, on 3 different Macs and over many OS versions, I’ve regularly gotten a message, “To continue to use VoiceOver you must download the most recent version…”, and then when I click through to do that, I’ve been unable to complete the update due to the inscrutable error -1701. And I don’t even know what VoiceOver does. Finally, after years of having to click “Cancel” at least once a day, Catalina has not once presented me with that dialog! Victory! Alas, on my old MacBook that I now use for media streaming to my home theater and haven’t updated to Catalina, the VoiceOver curse remains.
So I have recently priced up a new 16" MacBook pro which I was going to buy, and then I realised you can’t downgrade to Mojave. Thats it for me, I have 32bit apps and will never go to Catalina. I have ended up getting a high spec HP laptop with i9 and RTX 3000 graphics with Win 10, which is fine for the most part and my 32 bit cross platform apps work fine on that and Office is way better. I also saved about $1000 in the process. Apple’s loss.
Faced with having to migrate one way or another, I would have rather spent $49 on a Parallels license and stuck to running those remaining 32 bit apps in a VM under Catalina. I too get the impression Catalina is a total CF, but so is, IMHO of course, migrating to Windows. I have to use it at work for some systems every day and while it has no doubt made great progress compared to older versions, it’s still no comparison to macOS. Not even close.
That said, competition is a good thing and now that you have voted with your wallet I hope a) Apple gets the message (assuming you’re not alone) and b) you remain happy with your choice.
I’m trembling a bit after reading all this, as I’ve just ordered a new iMac. Had to, because important bits I need (principally Xcode) won’t work with my current machine which can only run High Sierra. There are trick ways of loading more recent OSs on my machine (mid-2011) but there is a real possibility of failure and the machine has so many other uses, especially during lockdown. I have a lot of emails (tens of thousands) so will have to archive them separately. Not feeling too good about this at all. Graham
My experience with Catalina hit a couple of rough spots in the beta releases so I didn’t transition my main machine until 10.15.4. The most significant reason for delay was the demise of iTunes which finally happened after the initial metadata loss with the advent of iBooks (now just Books) some macOS versions ago. An image backup of my iTunes library is waiting for a solution choice, perhaps a virtual machine with Sierra or Mojave.
Major third party suppliers seem to have 64bit applications and drivers well in hand. Some, like Canon, have abandoned support for perfectly functioning older peripherals. Others, like Brother, continue to support effectively ancient devices.
So, Graham, you should not tremble too much. Archiving your mails separately is a terrific idea. There are lots of products that can help. In the limited use I make of Xcode, no problems have bothered me. In the several macOS and hardware upgrades I have done recently (in-place upgrades, clean install and Migration Assistant, and, clean install and manual data transfer) major effort was in finding the new Apple default preference changes and resetting them as needed.
I know how you feel. I have a brand new 13" MBP arriving on Wed and I know there I’ll finally be forced to go to Catalina which I have so far deliberately avoided.
I have a very simple (and yes, perhaps naive) battle plan. I have plenty of backups, both TM, and clones, as well as keeping the to be replaced Mac on hand. I will be able to go back and retrieve Mail archives if that proves to be a problem. If I do indeed run into something I absolutely cannot get to run under Catalina I plan to get a cheap parallels license and run it in there under Mojave. Since I have a clone of a working Mojave system I plan to use that as a basis for this VM. Larger apps I use I recently already upgraded to make sure I have 64-bit compatible versions (Office, FM Pro, GraphicConverter, etc.). I want to believe I’m ready. ;)
But if anybody in here sees obvious holes, please point out my blind spots. I still have 48 hrs to prepare. :)
I’ve never tried this personally, since I don’t use Mail, but I have heard stories about people having trouble restoring from Mail from Time Machine in the event of corruption. Since it’s nearly impossible to separate Mail from your email hosting service, I’d recommend archiving your email out to a completely unrelated service (Mail Archiver X, EagleFiler, DEVONthink, etc), just in case.
Thanks, Adam. In fact, following what you guys reported in the other thread, I was thinking of actually migrating to the new MBP from a fresh SuperDuper clone of my old MBP. Have you heard such reports of Mail issues also with migrations outside of TM?
In any event, I plan on keeping my old MBP around for quite a while (before my wife probably takes it to replace her 2010 MBP since she’s too stingy to get a new MBA as I keep trying to push her to). 
If I run into Mail issues, I would hope to be able to simply grab them as an archive off my old Mac. Most of my mail is work mail and these days that’s all Google and their pseudo-IMAP so I hope I’ll be OK. My personal mail is also IMAP to my own personal mail server. I don’t have much mail archived in “On My Mac” where I understand most problems occur. Knocking on wood. But I guess cynics might just say that’s wishful thinking on my end.
I’m a bit nervous too. My new iMac is getting ready to ship. My concern is that my MBP is having problems and so I can’t complete a Carbon Copy Clone - it says it was unable to complete due to a problem - no indication as to what and Disk Utility essentially gives me the same thing, so I am hoping my TM backup will allow me to transfer my needed items. The mail thing concerns me as I am already having problems with Mail and I think it might be a major source of my difficulty. Sooo, will be holding my breath and doing a lot of praying! I find the posts here helpful though. Good luck to those like @Simon with their new devices!
Right back at you, @dougeddy. Sounds like you might need it. I realize that if you have a Mac on the verge of dying this whole transition to Catalina is definitely more nerve racking.
But see, that’s the problem. As soon as you launch Mail, it will try to sync with the servers, so any problems will be there too. Having the email On My Mac would in fact be safer, because it would divorce it from whatever another client can do to the truth in the cloud.
Oh, so I must have misunderstood entirely. I thought the problem was with local copies of email, not that Catalina actually screws up IMAP accounts.
If that’s the case I definitely need to archive IMAP emails first. Bummer. That’s a lot of work. Tons of folders, tens of thousands of messages with attachments. But thanks for the heads-up, @ace.
Yeah, sorry. Worth reading @mjtsai’s canonical post and the later comments.
https://mjtsai.com/blog/2019/10/11/mail-data-loss-in-macos-10-15/
I’ve spent more than a few hours over the last couple weeks with a friend in the UK who just bought a '19 MBP with Cat installed. MA from the 10yo ElCap MBP went well and pretty smooth.
Except Mail.
What a f’n mess. I think they are finally thru the worst of it.
Initially, they had a 60G Mail archive. Yea you read that right, 60G.
After plundering about and doing research on Cat (I’m on HS and will stay there for the time being. Been very stable) we managed to trim that back to 20G.
Still too big, but now it’s down to hand to hand combat. They know that’ll take time.
But seems like they are quarantined in the UK for some reason, so there is plenty of time.
Good, coz I was just about to suggest dropping back to HS if it would take it.
The removal of adjustable column width in Mail just defies logic to me.
But that new MBP is stupefyingly fast.
When I scroll their machine from here, it’s faster than my machine hands on.
There is something to be said for clock speed and 6 cores, or whatever it is.