No SSD/bootable installer won't boot
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?