Unable to reformat a partitioned hard drive for Catalina

steve626 wrote:

I think it is problematic to add a Catalina boot partition to an existing Sierra boot drive. The reason being that Sierra does not recognize APFS (it only recognizes HFS+),

macOS 10.12.6 Sierra is able to recognize an APFS volume, however, any older version of macOS 10.12.6, 10.13, and even 10.14 will have trouble understanding the layout of APFS volumes associated with macOS 10.15+. Even macOS 10.15 will have trouble understanding the Big Sur drive layout even though it is similar to Catalina's layout.

Users should not use an old version of Disk Utility when dealing with volumes/partitions created by newer versions of macOS since old versions of Disk Utility will not have a complete understanding of the new volume features. Just because macOS can recognize a newer file system/volume, does not mean all the other macOS utilities like Disk Utility have been properly updated.

In theory it should be possible to boot into the installer for the most recent version of macOS installer compatible with a Mac by using Command + Option + R which for a late 2013 iMac should be Big Sur, but a bootable macOS USB installer is the safest more reliable option.

Think possible solution is to Create a  Bootable Installer of Catalina.

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

Thereafter - do what needs to be done.

Is it possible that the Recovery Partition is still a Sierra Recovery Partition? If so, it won't recognize APFS and it might try to repair the APFS volume as appearing to it as "corrupted" with possibly disastrous results.

And what is that "edrive" on there -- is that a TechTool Pro third party recovery partition? Which recovery partition did you boot into? If it is a Sierra recovery partition, it won't handle APFS properly or even recognize APFS volumes properly.

I think it is problematic to add a Catalina boot partition to an existing Sierra boot drive. The reason being that Sierra does not recognize APFS (it only recognizes HFS+), but the Catalina boot partition must be APFS. So when you boot into Recovery for any reason, it will be Sierra Recovery (unless you can devise some trick to make and boot into a Catalina recovery partition), and Disk Utility in the (Sierra) recovery partition cannot recognize or understand the APFS Catalina partition. This means it will show it as "unknown" or "corrupt" when it might be fine.

I once asked the creator of SuperDuper about this and he recommended that the APFS boot partition (Catalina in your instance) be on a separate physical drive, e.g. as suggested to you by P. Phillips in their post that also provided instructions for how to go about this. In other words, keep your entire internal physical drive HFS+ (presumably Sierra), and create an external separate physical boot drive for your Catalina system. Basically what P. Phillips suggested. If you try again to have one internal drive partition for Sierra (HFS+) and another one for Catalina (APFS), whenever you boot into Recovery it will be the Sierra Recovery which will not recognize the APFS partition. There may be other unintended consequences down the road as well.

If someone here knows how to change your internal drive so it has a Recovery Partition that is Catalina, not its original Sierra, maybe he/she could pipe up here and explain how to do this. I don't know how myself without completely wiping the physical drive. If there is a way to create a Catalina Recovery Partition, that would for you work because Disk Utility under Catalina can reformat, repair etc. both HFS+ and APFS drives, but Disk Utility under Sierra can only work with HFS+.