Hi Everyone, I am new to the forum. I have a mid 2011 21" iMac. When I first bought the iMac back in 2011 I added more ram so for all the whilst I have had it it has had 12GB ram. I have recently been having some issues with it locking up on me. There is no one particular reason for the locking up, I could be doing anything, for example browsing the internet, all of a sudden the beach ball appears whenever the mouse curser is over the safari page. If I try to select another application it then locks up completely. I cannot access any other app or restart the machine but I can still move the mouse curser. It never fixes itself, I end up having to hold the power button in to turn off and restart. So far this has happened 5-6 times in the space of 3 months. The only thing that has changed in that time is I have connected an external HDD to start doing time machine backups. I spoke with apple and mentioned the external HDD and they asked me to run first aid on it which showed no problems. They then asked me to carry out the hardware test (Restart and hold D and then run test) which again showed no problems. No of course many of you will probably say disconnected the external HDD and see if it freezes again, which I will do, but for now does anyone have any other suggestions? I use my mac on a daily basis but have never carried out any maintenance or anything so maybe theres a few things I could try? Any help would be greatly appreciated. Oh and I am running the latest version of El Capitan.