iMac : Erasing hard disk

I wish to give away 2 old iMacs. But with the disc utility, I cannot erase the disc. The function ERASE is grayed out…. Is there a trick? It’s so simple with an iPad…

iMac 17inch PowerPc, 1Ghz (2003), ios 10.4.11.

iMac (21,5 inch, late 2009) 3,08Ghz, ios Sierra 10.12.6

Thanks

via Canadian Mac Forums at ehMac https://ift.tt/h3cume6