Mensagem original de: mayer ,
Not sure I really want to wade into this one. First remove the internal drive. Check your USB stick on another Mac to see if you can boot from it. If so, reinstall the 860. Hold down the option key on boot up and select the stick. Now go to Disk Utilities and format the drive GUID Mac OS extended journeyed. Now try to install a system from the USB drive. Do not use High Sierra as it will try to impose APFS formatting.