Trouble erasing an old iMac
I am trying to erase and discard an old iMac (late 2009, Intel), High Sierra. I have performed all of the steps in the Apple support instructions up to "Use Disk Utility to erase your Mac". When I restart the iMac and immediately press Command + R, a blank screen comes on with a gray Apple logo in the middle. Nothing happens after that.
Separately, if I do an ordinary startup and go into the Disk Utility and Macintosh HD, the Erase button in the menu across the top is shaded.
What should I do to completely erase this iMac?
Thanks