(define sixhat
  (λ (dave)
    (display 'ideas)))

where has my disk space gone?

I can't really come around in my mind that modern computers are so bloated with stuff (99% unnecessary) that free space just disappears before your eyes at alarming rates.

For many yeas I've been using some sort of treemapping app to visualize where has everything gone.

My favorite is OmniDiskSweepper. Takes little space, does its thing, allows you to act upon findings. What else would one need?