Initial Symptom: My husband's Mac had been randomly shutting down for some time, but we were limping it along. If your Mac is unable to access that feature, this may not work for you, but I included some other handy stuff, like recovering files from the drive. NOTE: This process only worked because the hard drive was still able to access recovery mode by holding Command+R on startup.

Please use Common sense if I did this and look around the screen for something that looks like what I'm talking about. I did most of this from memory, so I may have messed up a label here or there. This is probably overkill on instructions, but there are PC people out there who may try to help out the (much more) rare critical Mac issue. Hopefully I diagnosed a bad hard drive correctly (I'll provide my method) and I hope this helps someone out there. Being a 1 Mac household and an IT professional who has only supported PCs, I had a challenging time trying to figure out how to reload OSX after my husband's Mac hard drive died.

Replace mac pro hard drive