Hard drive issue on Imac fussion drive

Hi Jim.

How to fix a split Fusion Drive

If your Fusion Drive appears as two drives instead of one in the Finder, it's no longer working as a Fusion Drive. Here's how to fix it.

https://support.apple.com/en-us/HT207584

diskutil resetapple fusion join

Sounds like the HDD has died. If so then the options are,

1. Replace the HDD portion of the Fusion Drive.
2. Remove the HDD and replace the SSD with a larger SSD.
3. Buy a new iMac instead of wasting money to time on a vintage iMac.

Options 1 & 2 are fairly cheap fixes if you can DIY.

If not, a repair cost more than it is worth and option 3 is your best bet.