IF you can see the Mac HDD icon on desktop - or if its hiding - via Finder in Dock
use get info on that icon. That should open a new small info screen with both the
unused capacity and used capacity; these together may constitute the full amount.
In example follows is from another location: Total is Larger, & unused is Small number.
You could also choose to select in settings, for folder/drive to show its capacity.
This shows from Finder in Dock, in another window; as HDD is hidden otherwise.
Otherwise you'd have to look via Apple (icon) menu > About this Mac > (in new
window, choose More Info) then look for Hard Drives (SATA in newer, ATA older)
for one that will expose the working system's home startup drive. Because I'm
working with older G4 PPC Mac Mini 1.5GHz (Late 2005) in Leopard 10.5.8, the
screenshots I could take won't be a useful comparison, due to vintage difference.
Because one reply had been marked 'solved' and the path described is workable,
you have to know what the System Report (Profiler) tree represents &/or contains.
These internal names have changed; for those with more than decade of Mac use
this can be troublesome to just say where to go or how to get there ~ nowadays.
