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My Early 2008 iMac with OSX 10.8.5 no longer boots beyond grey screen with spinning gear. Verbose mode shows a load of I/O errors. Disk Verify/Repair run via recovery partition completes successfully, as does fsck terminal command in single user startup mode (it modified the files on first run but not second). I reset the NVRAM too.

However none of these have helped, same I/O errors afterwards. Thinking it might be due to OS files being corrupted I created a USB drive with installable version of OSX (actually 10.9 Mavericks), but it crashes during install.

I wanted to try Apple Hardware Tools diagnostics but the startup shortcuts D or opt+D don't work - I read it's been removed so presumably I'll need to find a way to reinstall it first. I've tried the usual fixes such as safe mode (won't boot), removing all peripherals, even the RAM sticks. I can't hear the drive making any grinding sounds.

Is there anything else I can try to fix this? Is it likely the HD has died even though Disk Repair is returning success?

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Disk I/O errors literally translate to input / output errors, which occur when the system is unable to communicate properly with the hard drive. This is very indicative of a hardware failure, whether it be with the hard drive itself, the logic board or the SATA data cable that runs between the two. Given the age of your Mac (5~ years) I would assume that the hard drive is to blame. Disk Utility repairs the volume on the disk at a software level, often it will report when hard drives have a hardware failure but there are also times when it repairs correctly despite communication errors still being present.

Apple Hardware Test (AHT) IS pretty good at giving you a solid answer but as you've found out it's no longer on your hard drive, likely lost during an old upgrade or re-install. You should be able to boot to it using the gray restore discs that came with your iMac. Discs from another iMac/Mac will not work unless it's identical to yours, these are very hardware specific. AHT should be on the disc labeled "Applications Install DVD", with a small mention of AHT and its version number. Holding down D while booting with that disc inserted should boot the Mac to AHT.

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  • Thanks, I found my disc and ran AHT, which gave the error code 4hdd/11/40000000 SATA (0,0) - looks like my HD is on the way out…
    – MachineElf
    Commented Oct 8, 2013 at 22:39
  • Sadly I would agree with that diagnosis. Thankfully it's a fairly easy repair!
    – Mr Rabbit
    Commented Oct 9, 2013 at 19:11
  • I get to go shopping for a new SSD drive so it's not all bad :) In your opinion what are the chances its not the drive that's died but the Mobo or cable?
    – MachineElf
    Commented Oct 10, 2013 at 14:51
  • @MachineElf, slim to none. I've replaced more hard drives in iMacs than I can remember (at least 100, probably a lot more) and can only recall maybe two times when it ended up being a failed SATA cable. You might place it safe by booting to a system before completely reassembling it just to make sure the drive is seen but I doubt that is even necessary.
    – Mr Rabbit
    Commented Oct 10, 2013 at 15:11

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