I can't help with the loss of storage except that if a HD states 1.5TB it never is 1.5TB it's always less
I have a 2TB and a 1TB - my total available storage 2.68TB
Regarding the previous subject if you want to protect your files give read access only and have only one account for these - if you have one family member who you trust that knows what they are doing then create separate account with read/write access. << I know where you are coming from.