The harddrive in my iMac (Late 2009 27”) died last weekend and I decided to replace it myself. Here’s some quick tips if you find yourself in the same situation. Not sure if disk errors are your problem? Boot you Mac and press “Command-V” during startup for verbose boot output. You’ll see messages about “Disk I/O Error” during boot.
There’s a great tutorial on iFixit that explains step-by-step how to replace the drive. I found my iMac had a 3.5” Hitachi Model HDE721010SLA330 SATA 3.0 Gb/s drive once I cracked it open. You can replace the drive with any 3.5” SATA drive you like. I chose to replace it with a comparable Western Digital drive that had more cache.
The original Hitachi drive had a built-in thermal sensor connector that the Western Digital did not. You don’t need a drive that supports the propriety Apple sensor. However, you will need to install fan control software like HDD Fan Control that monitors your drive temperature using S.M.A.R.T. and adjusts the fan speed accordingly. If you don’t want to pay the $29.99 for HDD Fan Control, there is also software from exirion that does the same thing (although I haven’t tried it). If you don’t have fan control software installed or the Apple thermal sensor, you’ll find that you iMac fan will gradually spin faster and faster until it maxes out at 6000 RPMs (and sounds about as loud as a vacuum cleaner).
Once you have the hardware installed, plug in your external Time Machine drive and power on your machine. Since there is no boot image on the new drive, the iMac will boot in recovery mode from the external drive. In Recovery Mode, use the Disk Utility to create a Macos Extended (Journaled) partition on your newly installed disk. Then use Time Machine to recover all the data from your backup. Be prepared for your fan to whine as the data is being restored. Once you install the fan control software (above), this problem will disappear.
My iMac is back to up and running except it runs noticeably quieter now than before.