under unixoid operating systems (like Raspbian inside the Onefinity Controller), there is no difference between a USB stick and a USB hard disk. The only diffence is that USB sticks are usually formatted as ‘superfloppy’ (no partitions) while a USB hard disk has a partition table with different partitions. Both work on the Onefinity controller and can be hotplugged.
If you want to export the connected hard disk to the local network, you could install samba on the Onefinity Controlelr and set up a samba server which can make a hard disk partition accessible as smb (windoze) share. There are tons of howtos in the WWW on how to set up a samba server and exporting a smb share under a Debian-derived operating system like Raspbian.
You can install samba on the Onefinity Controller this way:
sudo -i
apt-get install samba
and setup a samba server by editing the config file in /etc/samba following a howto available in the web (look at the samba docs)
But be aware that the Raspbian on the Onefinity CNC Controller is totally outdated and cannot be updated. Samba however should work, as long as your /etc/apt/sources.list.d points to the now archived Raspbian repository from 2017.