Can you hide/disable the internal SDD blade in an iMac (originally Fusion)?
I'd suggest this a hardware problem: you'd be best served by finding a local independent Apple repair company, and asking them to replace the SSD with a new/secondhand Apple unit.  

They'd be experienced at taking it apart and would charge a reasonable price. An Apple Store might say it's too old, or charge a lot.

The same modules are used across Apple's hardware of a similar age, so you should be able to get a larger size -- 512; 1Tb. (Don't Fuse with the HDD!) 

If your OS is on the HDD, that's massively limiting the performance of your iMac.