It appears such as you’re working every little thing – OS, apps, consumer recordsdata – from the arduous drive, and never utilizing the SSD in any respect.
If that is the case, then your Mac might be very sluggish. The entire objective of the Fusion Drive is to combine the pace of an SSD with the storage dimension of the HDD.
Nevertheless: your iMac is now 5 years outdated, and the common lifespan of a tough drive is … 5 years! There’s a excessive probability that the interior HDD will fail sooner or later. In the event you make a Fusion drive, and the arduous drive half fails, then the entire format might be ‘damaged’ — the SSD portion might be utterly clean (till you re-partition it).
Briefly, I’d advocate getting a USB 3 excessive pace exterior SSD, and abandoning the interior drives.
