An ext4 file system has its own internal Unix ownerships and permissions.
When mounted, those ownerships and permissions are what determine who
can read or write to each file/directory within the file system.
Yes, I know.
The ACL that's being added at the root directory of the mounted file
system is giving you extra read privileges on top of what the file
system permissions grant to you. I would ignore that for now.
The bad thing: These ACLs have higher pivilges than the filesystem and they
inhibit the writing of the device.
If you want to write to a mounted ext4 file system where everything
is owned by root, just become root. sudo cp myfile /mount/hans/whatever/
No good idea. For myself it would be ok, but not for my customers.
However, as I mentined before, I could either remove ACLs, or better just set
the directory to the rights of the user manually.
Howeverm the last thing is, what I would have been expected by the desktop
environtment. Thus there are two optins: Either this is a bug or it is set by
the developers to do as it does.
Maybe some maintainer or developer (who might read this) might know more.
Best regards
Hans