I have an iMac mid 2011. replaced the HDD with Samsung Evo 840 and removed the DVD and replaced with an OptiBay with an OCZ Vertex 4 After installing Yosemite and updating istat menu my iMac started to do sudden shutdowns or restarts. I am also using HDD fan control to fix the fan speed on the main SSD. Everything worked nice under Yosemite beta but now I am seeing this issue. I tried resetting the PRam and the SMC and nothing. This issue replicates very random. I checked the RAM errors with AHT and there is no issue there. I uninstalled iStat menu and everything works fine for now but I was wondering if anyone has similar issues. I googled this issue and I've seen similar reports on Mavericks but to me it started with Yosemite and iStat menu. Maybe it is a coincidence ... I don't know..