I swapped out a dead hard drive in a mid 2011 iMac 27", with a new seagate 1TB drive back at the start of the year. I also installed a fan control software to control the fan on the hard drive, which was all working fine. But my client has come back to me after they have only just started using the iMac recently. Basically after a bad crash while importing gmail into apple mail, the iMac no longer boots into the desktop but seems to be stuck in some sort of startup cycle. Basically the imac chimes on startup and I get a progress wheel(normal loading graphic) and also a horizontal progress bar that slow progresses from left to right but then seems to look like the imac is about to go into the desktop as I get a mouse arrow. But the mouse arrow then disappears and the screen basically goes back to the apple logo and spining loading icon but never seems to get into the desktop. So what I've done since, booted via external drive with system. Ran diskwarrior, reported it fixed drive. Reboot of repaired drive, still in some sort of bootup cycle with progress bar. Booted up via external drive again and started to backup problem imac drive using superduper just to be safe etc, halfway through backing up imac shuts down instantly. Tried this a few times and each time imac shut down with no warnings mid way through backup. Next thing I tried to do was update the current system on the problem drive with Mavericks, but at the end of the Mavericks progressing screen eg when its states computer will restart etc the imac freezes up and needs hard reboot. I'm basically stumbed as to what to do next, I have now backedup most of the important files from the problem drive just by copying files manually instead of using superdupa. Do I try formatting drive and try reinstalling a system, or is the hard drive poss defective?