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?
iMac Line (2012 and Later)