do not use Catalina but if you have already you must keep using the beta version as its the only one that can run diagnostic tools and read other file formats, this is because Mojave was not APFS and those versions of Catalina where able to convert prior formats to APFS using volumes or partitions of drives, now they are written for drives soldered on motherboards using only APFS, if you try adding anything the system will assume your modifying the computer Apple sold you and there for brick it., bricking the HD soldered on your motherboard means your mother board but on ext drives its tricky.
Example, failed Catalina installs on drives will still contain the data, only EFI diagnostics will not see the files, I tested this myself, I upgraded to the latest Catalina on a ext SSD, everything looked fine until I rebooted, once this took place I was left with no choice but access to the system of my internal HD, a 250GB SSD that came with the MacBook Pro signed certificate to that machine (you see even the parts are linked to the machine, changing anything means its bricked) anyway after I restarted the external SSD was empty!, nothing, a 512GB drive with my oder version of Catalina gong!,
6 months later I decided to see if I can get this external SSD to work with my iPad Pro using a camera adapter, and it did, but still refused to show any files on the Mac, so I backed up everything on my iPad Pro which has 512gb, tried re installing Catalina from a download I had since Mojave (older version) and it worked again! the files re appeared...
so anyone telling you that you have to reformat the drive is wrong, the files are there, Apple knows it, their techs know it, other critics of Apple also know it but they all want you to buy a new drive and spend days trying to recover files or replace everything, but it was clear in writing, Apple will not support older machines because of the component strategy, what's wrong is apple has not made it publicly clear which version of Catalina do or dont support certain machines but this is solved now with their latest greatest.
So your probably wondering if you should return to Mojave, its a good idea, only sad part is not having the features of notes and reminders or newer apps., it maybe worth it if it works for you, that is if you can deep format your drive, reset the pram and also leave your computer off and unplugged for 15 minutes, if you succeed its probably the best, other wise buy a new Mac...