slow Mac with spinning wheel!

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community

It might be more cost effective to get an external SSD, clone your internal boot drive to it and boot/run from the SSD. Then you can take the external SSD with you when you move on to a new Mac. That is unless your iMac is still under warranty or AppleCare.

If you could, please upload the report. There's a good chance the drive isn't actually failing. There could be intensive software in the background that's causing strain on your Mac. Plus, if your Mac utilizes only a hard disk (not a Fusion Drive or SSD), instead of replacing the hard disk, you may want to consider using an external SSD as the startup disk. It will make your Mac faster than any hard disk could, and the solution can be had for under $100.

Jack