Upgrading to Catalina possible?

Frans Brouwer wrote:

I have tried to upgrade to Catalina with my Mac Mini late 2012 with I7 and 1 TB fusion drive an 16GB internal memory.

Are there more Mac Mini owners with the same results?

No. My 2012 Mac Mini with an SSD and 16GB of RAM was only a little slow for the first day, while Catalina worked on it's initial background tasks.

My Timemachine backup didn't work.........

Please describe in more details what external drive your Time Machine backups are on and why you think that it is not working.

The upgrade took a whole day and when restarted the Mac Mini was useless - none of my applications nor finder where responsive.

Nothing helped to regain the feel and speed of my Mac Mini.

So I reinstalled the whole system to Mojave which took me a week to get all my data and applications back.

If you still have the original HDD in that Fusion Drive? Then I would suspect that either the Fusion Drive is split or that there is a problem with the HDD part of the Fusion Drive.

One way to get a better idea of what is going on with your hardware and software is to run EtreCheckPro and post back here tithe the full Report. see > https://apps.apple.com/us/app/etrecheck/id1423715984?mt=12

FWIW I dual boot my second 2012 Mac Mini with an SSD and 16GB of RAM, it runs Mojave on the internal SSD and Catalina on an external USB 3.0 SSD.