I wanted to update my sister-in-law's OS to El Capitan but there was a warning saying the hard drive was failing. Sure enough, Disk Utility confirmed the problem, so based on suggestions from iFixit, I ordered a 1TB Seagate hard drive from Other World Computing to replace the original Seagate 500 GB hard drive. Since my sister-in-law's computer only had 2 of its four memory slots filled, I also took the opportunity to order two more 2GB RAM chips.
The repair went without a hitch. I followed the instructions from iFixit to first put the new hard drive in an external drive case (USB 3 connector) that I bought from Best Buy and format it using Disk Utility from the original hard drive running Yosemite. I downloaded and installed El Capitan on the new hard drive, then rebooted while holding down the Option key so the new hard drive would be the start-up disk. I used Migration Assistant to transfer all the programs and files from the old internal hard drive to the new drive running El Capitan. It worked great. At that point I took the new hard drive out of the temporary case and installed it in place of the old internal hard drive using the iFixit instructions.
Two things in particular:
1) Migration Assistant did not show a progress bar or a time remaining indicator. I only had 70 GB of programs and files to transfer, so it only took an hour or so. But it is a bit frustrating not know where you are in the proceedings! Be patient!
2) The iFixit instructions warned of a potential problem with the thermal sensor for the hard drive which could lead to the cooling fan being turned on full blast all the time. Long story short, the new Seagate hard drive would only allow that connector to clip in one way, so there was no option to reverse it anyway. It works fine, so I presume that sensor problem has been attended to in the newer hard drives.