I updated to LM15.
Then I followed this post: https://wiki.archlinux.org/index.php/US ... ge_Devices.
But /etc/tmpfiles.d/local.conf was not present, neither the folder.
So I created both and I filled the file with the suggested line. I fixed the permissions (rwx/r-x/r-x)
Now I can copy to USB sticks with 3MB/s speed: it can be still quite slow (not sure if it is a 2.0 or 3.0) but at least the progress bar follows the transfer and I have no long wait time at the end with everything stuck.
Don't know if this can work also with LM13.