Catalina on an iMac with an HDD - resizing the Catalina container?

Just wanted to let you know that it all worked out fine. I basically followed the instructions in Apple's How To Erase a Disc for Mac although not all the steps.

First, I deleted the Macintosh HD - Data volume (just steps 1-4 under Erase a Startup Disk). From there, I skipped to How to change the partition map (scheme) of a disk because I wanted to do a 2-pass secure erase of the entire HDD. The key is to select the very topmost level of the physical drive, otherwise you cannot do a secure erase. Also, I think the naming example given there is misleading - whatever name you enter is going to become the name of the Macintosh HD volume in the APFS container - it is not going to change the name of the physical drive. I named mine iMac HD. After completing the secure erase, I had an APFS Container with 1 volume - iMac HD.

I then reinstalled Catalina and ended up with the APFS Container having two visible volumes - iMac HD and iMac HD - Data. (While the name is the one I gave it earlier, you should have both an HD and and HD - Data volume after reinstalling Catalina.) Then I restarted the iMac and everything was operational as if the iMac was reset to factory defaults.

After I was confident all was fine, I shut down the iMac and went back into Recovery Mode > Disk Utility. I again selected the topmost level of the physical drive, clicked Partition, added a second partition, sizing it to 750GB as Mac OS Extended Journaled. Doing this also resized the APFS partition to 250GB. Then I let it run. When it was done, I did a restart back into Catalina.

I now have a 250GB APFS partition with Catalina installed there plus a 750 MacOS Extended partition for my data.

Thanks for the reminder. This is a refurbished iMac that I have not yet used for anything, so I'm not concerned about backup at this point. I'm just setting it up. (That said, I do have a regular data backup scheme I use with all my systems.)

I plan to report back on how things went, including some screenshots. (An important note - I did this by booting into Recovery and running Disk Utility there. I suspect it would not have worked if I had booted directly into Catalina.)

Things actually went quite well once I realized that the APFS Container is actually a partition on the HDD. That only became apparent when I got past Apple's warning banner and opened the Partition window in Disk Utility. I added a second partition, resized the APFS Container to 250GB and formatted the new 750GB partition to Mac OS Extended Journaled.