Right now, your Mac can only boot because the USB drive is plugged in. Let's fix that.
Before you start the jailbreak process, make sure you: jailbreak macbook pro 2012
Re-boot your Mac, hold , select the internal EFI Boot , and you can now safely unplug your USB drive. Right now, your Mac can only boot because
By taking control of your 2012 MacBook Pro hardware and software, you can keep a classic machine out of the landfill and on your desk for years to come. If you want to start upgrading your machine, let me know: Which do you have (13-inch or 15-inch)? Which macOS version are you currently running? Have you already upgraded the RAM or storage ? make sure you: Re-boot your Mac
This can take a few different forms: