The folder "System" can't be moved into one of its subfolders. ***WTH?***

[eppur_se_muova]

A couple of years ago, I picked up a used iMac at auction for very little $$$. It came with a 650GB Fusion drive which I modified to have four partitions, only one of which boots. I deleted unwanted files from the boot partition, changed passwords etc., but didn't reformat that partition. Later I upgraded the OS with the OpenCore patcher to 10.15.7 aka Catalina. I've been using it with few problems until now. This is the first disk I've had that was already partitioned as APFS when I got it, and I'm seeing strange behavior I've not encountered before.

I tried installing two different Linuces on one of the other partitions, and neither could see the APFS partitions. When booting into OSX (from an APFS partition) I couldn't see the Linux partition(s) either ! This makes data transfer a little difficult. (APFS has been out for quite a while -- why can't it be read by other OS's ?? And why can't OSX read older formats for backwards compatibility ? This seems like burning bridges in a serious, totally non-advantageous way.)

I've backed up OSX with TimeMachine so I can do some serious disk cleanup without losing files to accidents.

I copied all the folders -- Applications, Library, Users -- from the OSX boot partition to a partition on an external USB SSD and then tried copying the System folder over. These are two *different* partitions on two *different* storage devices, both of which I am the owner. I should be able to move or copy any files I want to, maybe with a request for password. Instead I get the baffling message in the title:

The folder “System” can’t be moved into one of its subfolders.

The same thing happened when I tried to copy it onto a thumb drive (exFAT formatted).

How can a separate disk AND partition involve a subfolder of the folder being copied ?? Is there a workaround that doesn't involve reformatting everything first ??