trying to wipe hard drive - get unmount disk error
I am trying to wipe my Hard Drive as I would like to trade in my iMac. I have followed all the instructions, but when I get to the Disk Utility step I get a "Couldn't Unmount Disk" error. I don't quite know what else I need to do to resolve this issue?
iMac 21.5″, macOS 10.13