Preparing to upgrade to Mt. Lion, I was researching the steps. An article on MacWorld pointed to this article, to get a disk-space analysis tool: Byte breakdown | Macworld I thought Grandperspective looked cool GrandPerspective so I downloaded it. Grandperspective gives you a neat graphical representation of your largest files. Here's mine: See that HUGE block in the top right? That's iPhoto jpg library. I might have to rethink my iPhoto strategy. But see the three large yellow blocks below the red one? The largest is my LR 4 catalog. The next two largest are my LR3 and 2 catalogs. Still chewing up disk space to the tune of 3GB and 4 GB each. Then, the larger groupings of red and yellow blocks just to the left are something like 10GB of LR backups stretching back to 2009! I also found 2 1/2 GB MTS file that LR had lost in it's temp files somehow. I don't store video in LR, but sometimes accidentally import them off my SD cards. This one got disconnected and was just chewing disk space. The other thing I learned -- video, which I shoot too much of and almost never use or view, is pretty much the entire 1/4 of the left side in the diagram. Any rate, thought this might help long time LR users. If you do find LR elements that are chewing up disk space, please Google search to see if you are OK with deleting them. Don't delete something and then blame me for it, lol. Be careful and research, but at least this gives you some things to look for.