My 5yo iMac had become un-usable. I could never identify what was wrong with the hard drive but that was definitely the source of the problem. Plus I was running out of space on the drive
Replacement of the drive went very well (no swearing!) and everything is running well.
Boy was there a lot of dust inside the machine! It would have helped to be prepared with some compressed air to clean out the mess.
T6 Torx Screwdriver
Heavy-Duty Suction Cups (Pair)
TR8 Torx Security Screwdriver