Q: iMac Hard Drive Failing

Of course the hard drive can be replaced. Either at an apple store or service center. It won't be cheap.

Personally, I wouldn't attempt this because of the way the screen is attached with adhesive. Next, you would have to find a compatible drive--not sure if this is possible after-market, since I think they are Apple proprietary, and not available from Apple to the consumer. If you use a non-prop drive, then the fan will rev up to max. You might be able to deal with that by using the free http://exirion.net/ssdfanctrl/

Not sure if this is your model exactly, but gives a pretty good idea.

https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+EMC+2638+Hard+Drive+Replacemen t/28781

More videos of 'how to' below.

Other World Computing

This model doesn't have that issue with the temp sensor so a good brand name hard drive will work. Could upgrade to SSD. For the model that did have that issue an adapter cable from the aftermarket (ifixit) is available. The take apart is not for the novice or at least someone who doesn't feel comfortable doing it. The most important part is preparation and going slowly but it is not overly complex just looks like it.