Late 2009 iMac has a new lease on life
Failed hard drive. Late 2009 iMac is now "vintage", so Apple won't perform repairs on it. Because it was so old, i didn't want to invest a lot of money in it. I decided to try to do the repair myself.
It went exactly as described in the guide. I replaced the standard 7200 RPM 1TB Seagate drive with a 2TB SSHD Hybrid drive. Noticeable performance improvement.
When removing the ribbon cable (see picture), my tweezers didn't provide enough grip and ended up slipping on the insulation. To avoid damaging the insulation, I switched to needle-nosed pliers. They worked perfectly.
iMacs seem intimidating to work on because of the non-obvious location of fasteners. Don't be intimidated. Just use the right guide and take your time. With some iMac models, the same brand of hard drive must be used for the replacement. This is due to the proprietary connectors used for the temperature sensor. Not really a problem as it will be either Western Digital or Seagate. The problem for me was figuring out which brand was in there. The hard drive failed completely and wouldn't boot any way shape or form (e.g. to Utilities). I didn't want to open the iMac up just to figure out the HD brand, close it up, buy the new HD and then open it up again. Instead, i took it to a local Apple Store where they were able to perform a network boot and scan the hardware. I realized afterwards, I probably could have done the same thing myself with a USB boot. To restore my TimeMachine backup, I created a bootable USB El Capitan installer.
Other advice. Make regular TimeMachine backups. You never know when a catastrophic failure will happen. I was able to focus on fixing the hardware and didn't spend any time worrying about how to recover my data.