macOS Catalina - can't mount external hdd - verification/reading of the omap object failed: Illegal byte sequence
I had the same problem (invalid o_type (0x40000003, expected 0x4000000b)) after an upgrade of an OS to 10.15.3 and I was able to fix it in a very bizarre and unexpected manner. I added another APFS volume to the container that had a volume that was having problems. After that I was able to mount the old container with no problem. It seems now I have two disks sharing the same container and the same space, but at least the error is gone.
