Sometimes the hard drive fails to mount because of various reasons and prevents you from using the system. Macintosh HD or Mac hard drive is the primary storage medium for Mac users. Part 4: Tips against Macintosh HD Not Mounted.Part 3: How to Recover Data from Unmounted Macintosh HD?.Part 2: How to Fix Macintosh HD Not Mounted.Part 1: Overview of Macintosh HD Not Mounted.