I have a 27" 2013 iMac. Because it's a 1 tb spinning hd, I use a 512 gb crucial m100 ssd to a seagate external thunderbolt adapter for my boot drive. Been flawless the first almost two years now. Recently I've noticed it'll now randomly just reboot. I'll be in basically any program, or on the internet like safari or chrome. Sometimes I'll be on YouTube and it lets out a screeching noise until I manually hold the power button. I did upgrade to El Capitan about 4-5 months ago. So I'm thinking that's when it started happening. I've read some basic troubleshooting. Maybe reset things like smc, pram, pmc? I haven't really started troubleshooting it yet. Maybe start by using the other tb port but now it'll run for a few days give or take between reboots. Ideas?