I am experiencing a hang on OS X startup that has begun recently when rebooting. According to the system log, the hang is occurring at the line: com.apple.kextcache Created prelinked kernel /System/Library/Caches/com.apple.kext.caches/Startup/kernelcache_i386.18116732 . Prior to that line, I have the following messages: (Message shown 7 times) /System/Library/Frameworks/CoreServices.framework/Frameworks/Metadata.framework/Versions/A/Support/mdworker MDS Error: unable to create user DBs in /var/folders/zz/zzzivhrRnAmviuee++++KE+++3Y/-Caches-//mds (Parallels software, message shown 2 times) com.apple.kextcache prl_usb_connect.kext is not valid. The hang can last ten minutes, forty minutes, an hour, or never recover. After the gray screen clears, the hard disk begins clicking rapidly, as if loading something, but then it goes quiet and the startup process hangs. The computer services are running in the background, as I can hear the hard disk clicking occasionally, and the screen blanks as if the Energy Saver functions are active. Sometimes the keyboard backlight comes on, sometimes not; sometimes the mouse cursor appears, sometimes not. One time I left the computer running overnight, and hours later I found it stuck halfway between the wallpaper appearing and the login window appearing, the screen showing an opaque gray over the wallpaper but no login window. I have tried to delete the files in the com.apple.kext.caches folder and let OS X recreate them, but that didn't fix the problem. I'm guessing that there's something wrong between the prl_usb_connect.kext and the creation of the kernelcache_i386... file. In /System/Library/Extensions , there is something suspicious. There is one file that's not a file, but a Shortcut, and that's associated with prl_usb_connect.kext . The Shortcut points to /Library/Parallels/Parallels Service.app/Contents/Kexts/ . Neither the Shortcut nor the .kext file appears to have been altered recently. Should I delete prl_usb_connect.kext, delete the Shortcut to prl_usb_connect.kext, delete the com.apple.kext.caches files, and try again to let OS X recreate the files? Should I clear the files in /var/folders/zz , including the folder zzzivhrRnAmviuee++++KE+++3Y that is mentioned in the logs? What may have caused the problem to occur?