Do I have to reinstall the OS if I'm trading the Mac in? Can I just reformat it and not reinstall?

Thanks to William and Tesserax. The reason I had asked this question is because my first attempt at reinstalling the OS failed. The iMac had presented me with two hard drives to choose from: Macintosh HD or Macintosh HD - Data.

I chose Macintosh HD and 3.5 houirs later the final step of the install failed.

I called Mac support and a fellow said it shouldn't matter which of the two I used. But it DID. A second person (a lady) said I should have used the "Data" HD. I figured if the secomnd install failed too, I would just send it back (it's a trade-in to Apple). She said that would be OK.

But it worked! This morning the iMac had Catalina installed and ready to go. So it's on its way to some 3rd party company that handles Apple trade-ins.

Thanks again.

I was surprised to see two "drives" on the list. When I reformatted the HD, it had those two partitions. Now that I think about it, I wonder if the "Macintosh HD - Data" partition was created when I had Parallels running a few years ago.

In any event, I reformatted both partitions separately, because the beast appeared to only want to do one partition at a time, instead of reformatting the entire drive.

Then when the reinstall of Catalina OS on the Macintosh HD drive failed, I wondered if I should have used the "Data" part of the HD. As I mentioned above, that turned out to be the case.

In any event, UPS has the old fellow on their truck and its on its way to the trade-in company. And it was happily running Catalina when I shut it down before paking it up.

Thanks again to all who responded to my call for help.