How to | Deleting iMac (before giving it away, selling)

back up computer (eg. Time Machine)
Sign out of iTunes [open iTunes/Account/Authorizations/Deauthorize this computer & Deauthorize Audible]
Sign out of iCloud [Apple icon on top menu bar/System Preferences/iCloud/Sign Out]
Sign out of iMessage [open iMessage/Messages/Preferences/iMessage/Sign Out]
Sign out of google accounts
Sign out of Amazon
Unpair Bluetooth Devices (optional, and obviously don't disconnect bluetooth to mouse and keyboard that you'll need to use to continue the erasing process)
Erase
- Shut down iMac
- Turn iMac back on, and immediately press & hold (Command + "r") until apple logo appears to signal boot up
- Select "Disk Utility"
- "Erase," choose APFS format. [this should've worked, but my disk had problems so...]:
  - in my case, "erase" was greyed out, possibly because my iMac 2014 had "Fusion Drive," an HD & SSD hybrid.
  - I had to separate HD & SSD [choose "Partition," then HD drive, and format: "APFS."] Splitting HD & SSD means no more ability to move freely from HD & SSD.
  - Still, "Erase" would not run, because something was wrong with the HD.
  - So, I ran "first aid" for HD & SSD.
Click "Erase"
Choose "Security Options," and move the bar to the "highest security."
Erase hard disk. This will take many hours.
When erase is complete, Quit Disk Utility
Reinstall macOS High Sierra OR:
1. Strangely, as a result of the partition, this 2014 iMac was able to install the latest Mac OS, whereas it could not do software updates since macOS Big Sur.