I have an HZCore streamer running ROCK.
I have 4TB of internal storage with music files I’ve loaded.
How can I see how much storage is currently used?
The ROCK web admin interface tells you
Thanks very much–I’m a noobie.
Welcome to the forum. Ask and you will be helped 
While @Suedkiez is correct, i am almost certain that you are not running ROCK.
ROCK is the name for approved NUC’s running RoonOS, while your HZCore is a proprietary solution.
The question should directed at the sales point where you purchased the beautiful server!
You might be able to see a hint while mapping the storage of the HZ ore over the network as well, but that could be misleading.
Edit: From what the allknowing Internet tells me it is actually running RoonOS, so it seems to behave very similar to a ROCK device:
http://hzcore.local (from a Mac or iPad) or
http://hzcore (from a Win-device)
It does use ROCK, so for your purposes at the moment you can treat it as regular ROCK.
(If it comes with ROCK preinstalled, it may be somewhat questionable from a licensing point of view because I don’t think the ROCK license allows redistribution as a preinstalled solution, except if the manufacturer has an agreement with Roon Labs. Though selling it without a preinstalled ROCK and letting the user install it should always be ok, I think)