This is really important.
If these are the original drives, and they have not been replaced, it is time to check them and decide if you want to go any further with a fusion drive configuration, and also if you want to continue using the iMac.
First, if you have anything already installed on this system that will report the S.M.A.R.T. data for both the SSD and the HDD, see what your utility reports about drive health and time in use. If you don't have a disk utility that will give you S.M.A.R.T. information, download the trial of DriveDX from Binary Fruit.
https://binaryfruit.com/drivedx
Next, go to iFixit and look at what it takes to replace the drives. The odds are really good that both of your drives are at or beyond their life span. A 2014 27 inch iMac is a bear to replace internal drives. If you want to keep using it, I would suggest booting from an external SSD.