OK, my HD is.... "messed up" may be putting it mildly. (Specs: iMac 27-inch late 2012, Mavericks 10.9.5, 8GB/1TB) I seem to have 8 total partitions, including several hidden partitions on my drive that I did not create. Two are "EFI," two are "Macintosh HD," along with "Boot OS X," and the expected "Recovery HD." As below: This came to my attention while trying to install BootCamp, which I was able to do successfully, as you can see. However, the extra drives seemed odd, especially since I get three (3?!) EFI drives showing up on my desktop briefly on reboot. (One of these may for my cloned copy of my drive? The items below "Manor" are all proper external USB drives, except for BootCamp.) (Full disk util list and diskutil cs list results posted HERE on the Apple Forum) , so I won't duplicate them here (I can if desired, of course!). Things have been working fine, despite the oddness, so it wasn't a problem until I realized every restart now defaults to the Windows partition. I assumed I had left it that way in the System Prefs, but when I went to change it, I got this: I can boot into OS X, but only if I shut down and restart with the Option key and select the main OS X HD at the boot volume. My thought of course, was to wipe the whole thing and start over. I am thoroughly backed up, and am willing and able to nuke the entire drive and ALL its partitions BUT Disk Utility won't let me. Under no set of circumstances I have tried (external drive boot, OS X DVD, or Recovery Mode) am I able to repartition the drive; the options to do so are greyed out. I have seen some *similar* issues with Yosemite, but I haven't touched Yosemite, and I have yet to see this many extra partitions. AND, none of them list as 'revertible.' Help?