Hello, The other day I was performing routine maintenance on my Mac Pro using TechTool Pro. I decided to run the tool "Volume Optimization" overnight since it takes awhile to complete. When I returned I found that my standard start up disc, the internal HD would not mount, which then forced me to start up from the OS X software disc. I ran the verify disc and repair disc tools in Disc utilities only to find out that the HD is fine. But, it still won't mount. I can even run TechTool Pro services on it, but it still won't mount. Any help would be greatly appreciated as I have spent the last several hours researching causes and solutions to my problem, but haven't found anything that will work in my particular situation. Thanks again!