I have a 2010 Mac Pro with a 1 TB boot drive and a 2 TB second volume. My OS is Snow Leopard. I run a weekly clone of my boot drive. The last time I ran the clone was Monday, 4 days ago. A check of the clone drive today shows 684.39 GB used. However, a check of the boot drive today shows 922.22 GB used! I opened up a Finder window for each hard drive and did a Get Info on each folder that shows up on the root volume of each drive. The difference for each folder between the two is negligible--less than 1 GB. For example, the Library folder takes up 12.69 GB on each drive. The Applications folder takes up 25.99 GB on each drive. The Users folder takes up 215.27 GB on the boot drive and 215.73 GB on the clone drive. The are no folders that show up in Finder on the boot drive that doesn't also show up in Finder on the clone. The Trash on the system is empty, showing zero bytes on 57 items. I don't run Time Machine. I use Carbon Copy Cloner about once a week to create a complete clone of the boot drive, so they should be identical in size. So where did the 237.83 GB go? There appears to be some hidden files somewhere, but I don't have the faintest idea of where to look for it.