Did you read my post from above? If you plug your APFS disks in an iPad Pro iOS can see the partition and you can copy your data safely and reformat the drive. If you need more assistance let me know.

Hi everyone, Today I was surprised by my MacBook with the same problem. I have the 4TB Sandisk Extreme on USB-C and I got this error. Tried a few things and by the time I have decided to give up and getting ready to delete the messed up partition and format my drive, I decided to give it a try on my iPad Pro Gen 4. Guess what. iOS can read the partition and right now I am in the process of copying everything of the Sandisk. This is what works for me. I can see all the files and I am more than half way with copying. Have a good day. Uncle E