I need to reset my admin password (https://apple.stackexchange.com/a/61932/17533). Can't boot into Recovery Volume, so I need to boot into Single User Mode and modify the Data volume.
When booting into Single User Mode, the system shows the following help:
To mount the root device as read-write:
$ /bin/mount -X /
However, that leads to an error message:
mount: illegal option -- X
I also tried the old
mount -uw -, but that leads to some firmlink related messages, and afterwards I still have no writeable Data area.
mount shows that the Data volume is already mounted at
/System/Volumes/Data as read-only. Trying to unmount it with
umount leads to an "Operation not permitted" error.
mount -uw /dev/disk1s1 (which is the Data partition as shown by
mount) leads to the error "Found 2 volumes in group ... Failed to stat file //AppleInternal, error No such file or directory".