I am fairly new to Mac. I bought a refurb 27" 2011 iMac a month or so ago. I am having issues with shutting down or restarting. Sometimes the iMac just won't do either. I have a 3 TB external HD formatted to NTFS so. I purchased and installed Paragon for OSX 11. Sometimes, when I click on that drive I get the spinning beach ball. If I do a comand-alt-esc and try to restart a non-responsive finder, it quits but does not restart. When this happens and I try to restart, the imac hangs on the gray screen during shut down. Is there a way to troubleshoot this?