1. Root can change the ownership. It's as simple as that. There's no 'operation not permitted' for root regarding this, basically.
2. By 'clean' I meant if the device is already used or some condition like this. You printed it in your reply: "fuse: mountpoint is not empty". Eg. it's not clean.
The kernel has nothing to do with this. This is much beyond the kernel. You can argue that the filesystem is part of of the kernel and yes it is, as far as providing the basic services. The management of access is something else unless your are using SELinux.
To see which drive are mounted and where and at the same time to which capacity they are used, use:
I don't do any diffs with the backups on home systems. Otherwise each backup is done in a folder whose name has the date and time. In ambiguous cases where diffs have to be made, it's only a few text files and that's easily made using for instance emacs. There's no clunkiness in that for the simple reason that it's avoided from the start.
"rsync error: some files/attrs were not transferred "
As I've mentioned, if I would get that kind of error code in my script from rsync and stop the backup then it would be my own (lucky ?) decision. Otherwise this kind of error does not stop rsync.
The other errors such as: "rsync: opendir "/home/z/.dbus" failed: Permission denied (13) " are simple to troubleshoot. Just try them manually and see what the problem is. To do so you first must know how lucky runs, as root or as user ? Then try the same by yourself. This ~/z/.dbus looks strange anyways. Who is user 'z' ? Looks like there's an attempt to open a communication channel within dbus. I do not run dbus on Xubuntu 18.04.
The others like "rsync: chown "/mnt/ff04ca1e-58e7-4ac2-92b5-78fd02007bc9/1. ..." Just try them. What is the user given as argument to chown ?
Making sense of them is easy. There were attempts to change the ownership of a few things and that was not allowed. Duh. Root can change ownerships. Does lucky runs as root ? What if you try it yourself, first as user, then as root ? Why is there a change of ownership in the first place ? Because rsync is run with the -o option ?
Apart for music purposes, document editing and web browsing, I run no GUI really.
Anyways, looks like you're not the only one having that kind of problems with lucky.