Disk utility ERASE grayed out on 2009 iMac that I'm trying to recycle
Do I need to start up from a CD, or is there a way I can erase the hard drive if it's the start up disk?
Earlier Mac models
You cannot erase a drive you are booted from.
Boot into Restore mode,CMD+r keys...
Look at top of screen for Utilities menu...
In Disk Utility>View, select Show all Devices, highlight the top left entry.
Did you do these???
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support