I also recently connected an HDD to the Freedombox and had the same problem you describe with the software cockpit.Although I don’t know if it is correct, my solution was to create a new account in cockpit.
To do this, go to the cockpit interface (Accounts> Create New account)
After you have done this you have to check “Server Administrator” and then login with the already created account.
Formatting, Partitioning etc are now possible.