I have changed my CCC backup from a 500 GB drive to a 1T drive because I was running out of room. I now wish to re-use the 500GB drive but am unable to erase it.

Using Disk Utility, an attempt to erase gets this message:

Preparing to erase APFS Volume content
Checking mount state
The volume "OWC 500" on disk6s1 couldn't be unmounted because it is in use by process 0 (kernel)
Couldn't unmount disk. : (-69888)

Operation failed...

Leaving the drive attached I shut down the iMac and, after restart, tried again. Got the same message.

Thoughts.

Nevermind....

I booted using Recovery (Command-R) and was able to erase successfully using Disk Utility.

For future reference, I can't date it, but for a while, now, I've run into failure notices when trying to erase a drive, but in every instance, simply rerunning "Erase" immediately has been successful.

It's also my experience that simply repeating the request will usually generate a result, but not in this case. I gave it a try two or three times but my machine was having none of it.

Be sure the drive has been removed from Time Machine before attempting to erase it. IIRC this is a fairly recent protection added to TM drives.

In this case, I don't think the TM drive would have been an issue as I was replacing the Carbon Copy Cloner drive. However, when drive changes are involved, I have always had a practice of going to "Shutdown" first.

Special Mention:

I had wondered about keeping my old Carbon Copy Cloner data and popped a quick note to CCC, advising how I thought I might do it. They were quick to respond with a "We don't advise that way because..." and provided guidance on the right way. Pretty good service.