How to know if my SSD is still working in iMac fusion drive?

In System Information, select "Storage" from the left0hand contents pane to find the mechanical part of the Fusion drive.

This Apple article shows how to tell if a Fusion drive is "split."

How to fix a split Fusion Drive - Apple Support

Instead of opening the Mac for the SSD, I would suggest just getting an external USB SSD and run macOS from that. It will be just as fast as a Fusion Drive, if not better. It will be around 420 MB/s read/write. If you're interested in this, please read: How to Setup and Use an External SSD as y… - Apple Community.

Cheers,

Jack

Hi thenuskot,

I don't think the "Split" Fusion Drive issue applies to you, as it is not being identified in Disk Utility or System Information. It just has a generic "Apple SSD Controller" rather than the actual SSD.

Cheers,

Jack

Then there is some hardware fault with the SSD.