I just got a new Raspberry Pi 3 Model B. I am trying to set it up and when I plug it in, am only getting the rainbow screen with the little square on the upper right-hand side of the screen. From other documentation I've read, it seems that this is a too-low power supply issue. I currently have my pi plugged into a wall outlet. I would believe this, however I just ran a different pi and it worked just fine.
I believe this is actually an issue with the SD card. I have used this new pi with an old SD card and it worked just fine. With this card, I also added "boot_config=1" to the config.txt file, but I still can't get anything to happen with this new SD card.
Any ideas what might be going on, or ways I could attempt to fix it?