mac mini 2018 cannot boot to startup manager
trying to boot into startup manager for booting from other disks via usb and thunderbolt using new mac mini 2018 running Catalina. If I hold the alt | Option key as stated the mac never boots up, stays on black screen. Booting to recovery mode works fine with cmd-R but option key just halts boot even releasing the key boot wont continue unless I hit the return key several times but then just boots into normal mode finally once I do that. I have a USB keyboard plugged in to bypass blutooth keyboard issues at bootup, moshi for apple and have disabled secure boot and all that in T2 chip as required. WHy does option key halt booting ?? screen stays blank can see no messages until apple logo appears with thunderbolt monitor.
Mac mini, macOS 10.15