Pantheon Files ignores higher group-permissions

since eOS 5.1 is fast as hell, I finally use it as my main OS.

But: when ever I mount my home server, I cannot manipulate files on its external HDD.

Problem: all files on my NTFS-formatted HDD are owned by “www-data”, and have the group “exthdrw”, which I am member of. All files have permissions 460, so I can read/write files via my group membership.

But Pantheon Files ignores tgat. I cannot copy files onto the external server HDD.

It works in Caja (MATE) and Nautilus (GNOME).