USB card reader causes system to lock up

So here I was, ogling over my newly built system. I put my phone's SD card in, copied a few files and shut the system down. When I started the system next day, poof - system locks at the BIOS sign-on screen. In the course of troubleshooting, I ripped the system apart, only to discover that I had left the SD Card in and the system was set to boot from USB devices. Took the card out, entered BIOS and changed Boot Sequence to HDD first. Fixed.