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I’ve got a 13” macbook pro A1502/EMC2835 mid-2015 with no SSD. I am planning on buying a replacement SSD, but I wanted to check if it’s OK first, so I used a USB flash drive to create a bootable installer (Mojave), connected it to the mac, pressed the power button and the option key, and I see a flashing question mark in a folder icon. I see this means the machine isn’t finding the USB flash/bootable installer drive. My question: should it find the USB flash/bootable installer drive without an SSD installed? Or is there a way to test before I spend the $$$ for a replacement SSD?
It sounds like you didn’t prep the USB drive correctly. First, you need to use another Mac (you can also do it on a Windows system its just harder). Using the Mac you need to first open Disk Utility to format the USB drive to GUID with a Journaled file system (HFS+), then following this guide How to create a bootable macOS Mojave installer drive.
