Samba master if mount with "posix" then FSCTL_SRV_COPYCHUNK_WRITE will return "STATUS_INVALID_HANDLE" where the same thing works fine if you mount without "posix". Client requests an access_mask including FILE_APPEND_DATA on the destination handle which lets us take the true path in this if condition if (posix_open && (access_mask & FILE_APPEND_DATA)) { flags |= O_APPEND; } so we open the destination file with O_APPEND. This then failes in copy_file_range() because O_APPEND is not allowed on the destination file handle.