Missing Storage Space (PR4100)

I have searched through previous posts for relevant information, but if I’ve missed something, and this is a duplicate post - apologies!

I have 24TB (4x6) of storage in my PR4100. Due to RAID configuration, this is down to 18TB due to my RAID5 setup.

When I browse to the NAS in Windows, I can only locate a total of around 5TB of actual data. But the MyCloud OS indicates I have 4TB of remaining space. What am I missing? What is eating up the mystery TBs?

Thank you so much in advance.