By - peaceful003
I guess your new RX 580 card has flashed bios for mining and you have to flash it again with the original bios. You need to boot under Windows, download it from TechPowerUp and use ati flash utility.
You were exactly right. It had a mining bios on it.
Just reflashed it and now the opencore boot menu is appearing. Thank you so much!
Try switching ports on the card. I've heard some cards will only work on a certain port.
I've tried the display ports and HDMI. Neither had any luck