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
Boot up in Internet Recovery mode on power on (option command r) to load recovery off the Internet. Then in the internet recovery Utilities go into Disk Utilities, click the "View" menu and select "Show All Devices" so you can see your "Apple SSD" or "Apple HDD" above the Macintosh HD, and then erase that Apple SSD/HDD and that'll work.
trying to wipe hard drive - get unmount disk error
