The hard drive in question was checked with disk utility for permissions or other issues before I ran defragmentation operation on it in hopes of improving performance. But after the application crashed on me, disk repair says "Disk Utility can't repair this disk." advises me to back up my files, and reformat the disk. But I cannot even erase it. The attached screenshot shows the only message it gives me. I am glad the hard drive didn't contain any critical data and was completely backed up with time machine. While this is the first time this has happened with me after tens of successful defragmentation, I have learned my lesson and will not be running defrag apps on my iMac again.