Personally I find treemaps hard to read, and for getting an idea of what's taking up space on a disk I much prefer pie charts like [1]. My favourite for Windows is Scanner by Steffen Gerlach[2]. I haven't found a good one for Mac yet.
[1] http://www.steffengerlach.de/freeware/scnshot.gif [2] http://www.steffengerlach.de/freeware/