I have seen it first on Xcode and then I downloaded a free app (Disc Space?) from the App Store. I have a dozen apps installed and 20GB are already gone? I understand the OS can take quite some space, but isn't it a bit too much? Is it possible that it's due to the many log files? I always delete the sysdiagnose outputs I generate (Volume Down+Play) but I see quite some logs are still there. It really makes me wonder if I didn't make a (huge) mistake not waiting for the 64GB at this point.