Fixing Disk Utility error -69877

Carbon Copy Cloner’s weekly backup task broke after my iMac’s upgrade to Catalina recently. I followed these instructions from CCC to reformat the external drive as APFS but Disk Utility kept crapping out with error “-69877: couldn’t open device.”

I tried different compatible formats but Disk Utility resolutely refused to reformat the disk.

Many searches for -69877 solutions yielded people who recorded numerous reformats, reinstalls, etc. I held off taking the complicated solutions seriously, though; I particularly ignored the worst-case solution: junk the disk. I figured if I kept looking, some stray sentence somewhere would ring my bell.

And it did. In one of many forum discussions about -69877 on the Apple site, I found this gem:

Sometimes when Disk Utility gets stuck like this for me, I choose a PC format to initialize the HDD then reformat with the Mac settings. So after you give it a name, click the first pull-down menu and select MS-DOS (FAT), then click the second pull-down menu and select Master Boot Record. Go ahead with the reformat. When it completes, just re-format again with the Mac settings.

I reformatted the drive as MS-DOS (FAT) and Disk Utility erased the entire drive. Success!

I was then able to follow the CCC directions to select APFS and create the CCC partition and an extra partition I want to use for storing large files.