iMac Hard Drive Churning, Constantly.

I recently upgraded my 24" iMac (early 2008) OS to El Capitan and I then started to notice the hard drive constantly churning. Over time, my iMac got increasingly slower and slower to the point where I would be seeing the beachball endlessly. Eventually after some swearing, I started to look into what was causing this. I found that Dropbox was one of my startup items in my Accounts < System Preferences so I deleted it. This helped. I then went into Spotlight < System Preferences and instead of deselecting various items in the Search Results window, I clicked on the Privacy tab and dragged my hard drive icon into it and any other drives I had mounted. So now spotlight won't index anything. The constant hard drive churning seemed to stop. I never really used Spotlight as I prefer using Easyfind. Hope this helps.

Run Apple's Hardware Test in extended mode 2-3 times back to back. If any errors appear take the machine into an Apple Store or AASP for immediate repair. The reason to run the test 2-3 times is AHT does not always pick up errors the first or even second time however when it does it is correct.

That usually happens when you don't have enough RAM so the system starts using space on the HDD for memory.

About OS X Memory Management and Usage

Using Activity Monitor to read System Memory & determine how much RAM is used

OS X Mavericks- About Activity Monitor

Understanding top output in the Terminal

The amount of available RAM for applications is the sum of Free RAM and Inactive RAM. This will change as applications are opened and closed or change from active to inactive status. The Swap figure represents an estimate of the total amount of swap space required for VM if used, but does not necessarily indicate the actual size of the existing swap file. If you are really in need of more RAM that would be indicated by how frequently the system uses VM. If you open the Terminal and run the top command at the prompt you will find information reported on Pageins () and Pageouts (). Pageouts () is the important figure. If the value in the parentheses is 0 (zero) then OS X is not making instantaneous use of VM which means you have adequate physical RAM for the system with the applications you have loaded. If the figure in parentheses is running positive and your hard drive is constantly being used (thrashing) then you need more physical RAM.

Adding RAM only makes it possible to run more programs concurrently. It doesn't speed up the computer nor make games run faster. What it can do is prevent the system from having to use disk-based VM when it runs out of RAM because you are trying to run too many applications concurrently or using applications that are extremely RAM dependent. It will improve the performance of applications that run mostly in RAM or when loading programs.

I was having similar situation with my iMac too. Something called SRM was running at the top of activity monitor list. I remembered that I had just put a folder of large files in the trash can. When I emptied the trash the disk activity stopped.

How much free space do you have on the disk drive?