How to reformat (erase) iMac with Fusion Drive. A copy of the data still on Non-bootable (old) iMac

Background

When using Migration Assistant to move data across the local WiFi, from a FusionDrive equipped iMac to a brand new machine, everything went fine but slow and then just hung up. The destination (new) iMac stopped incrementing the processed number. After waiting 20+ hours, I hit the ESC key to the destination (new) iMac and that Mac stopped cleanly.

I got the time machine disk (should have done this first) off the old iMac and moved it to the new iMac. I restarted Migration Assistant, selecting as the data source the time machine disk. Everything moved over fine and the new iMac is happily in use.

Problem

When I started to cleanup the old machine, to give away, I noticed that it would no longer boot.

I called Apple at this point. We did get it to boot by pressing command R. We tried the DiskUtility and Reloading the MAC/OS utility. We attempted this both using the built in recovery utilities and version download from the (Boot iMac with Option-Shift-Control-Command R.) both of which failed. I'm pretty sure the old iMac used up the small amount of available free space and then froze.

Question

How can we erase/reformat the old drive so that the machine may be safely passed on? Just curious, is this related to the Fusion drive or do all file systems respond so?