You say you left the fusion drive components in place, but, for some reason those internal drives do not show in Disk Utility. They should appear in DU, even if you are not using them, as long as they are still connected.
You can double-check to see if the system sees either of those devices:
Open your Terminal...
Type the command
Then press Return
you will see a list of all connected devices that are storage devices.
Your existing external SSD (the drive you currently use) will be listed as an external, physical device
You will probably see two devices listed as internal, physical devices
That's your fusion drive, and your Mac still sees them, and they are still powered up. The spinning hard drive still spins, because it is still connected, and it's a full-sized 3.5-inch spinner.
And, "unused" does not mean that it has no affect. If the spinning hard drive is still spinning, and fails, the result can be unusual problems, perhaps slowdowns of the system, including that dreaded spinning beach ball