As a Catalina beta tester, I filed this bug. Apple said there were more than 10 similar reports.
It has gotten a lot better, I only see the error occasionally so it might just be physical movement of the device.
But it used to generate that Not Ejected Properly error repeatedly just sitting on the desk and not being used.
My suggestion is to update to the latest release version of Catalina. Catalina is very stable at this point.
Work has shifted to Big Sur although there was a release of another Catalina beta build for Mac OS and iOS 13 just last week.
So maybe they are still working on some aspects of Catalina for a final public release. I am sure they want to make sure the transition to Big Sur is flawless.
Sierra to High Sierra release (not beta) on V1 update bricked a number of 2012 MacBook Pros. Mine included. The 2012 MBP was compatible with everything in High Sierra except the new file structure.
When the updater tried to rewrite the drive, the result was a bricked machine. Apple had to give those affected brand new free replacement machines direct from China in exchange for the bricked machine.
And V1.1 of the updater was released within 72 hours. I would love to hear the true inside story of that screw-up and if anyone got fired over such an expensive mistake.
I am holding off on testing Big Sur until the release notes are down to one or two lines. And I think I will load it on an external drive once I learn how. Because the beta build Apple sends doesn’t seem to allow choice of drive.
Maybe boot from an external drive with Catalina and it will update the startup external drive to Big Sur?