Playing with the RPi4B 4GB, as a temporary measure I plugged in a micro SD card, ethernet cable, wireless keyboard dongle, Haupagge tuner and a 128GB SSD.
Trying to start it up I get the initial boot screen then it just goes black. If I unplug the SSD it will boot correctly and I can then plug the SSD back in and access the videos on it.
I have a 3A power supply (from ThePiHut), and not only does the SSD work on the Pi I can find no problems on it when plugged into either a Windows PC or a Linux Mint PC.
I'm not overly bothered because I'll be plugging a powered 6TB drive in for real but I'm interested as to what the problem is. Any thoughts?