[beagleboard] BBB can't connect to laptop through USB port with the existence of SD card

After update Angstrong in eMMC by SD card, I restart BBB with and without SD card. Everything is fine without SD card and I can access to it through ssh in linux console. But if I leave SD card on the board and just boot up from eMMC, after log in as root, BBB would try to connect USB port but

[beagleboard] would a bad modified u-boot damage BBB to ?

I just got my first BBB and try to study u-boot on it. I just wonder whether or not some bad assembly code in charge of initializing/configuring hardware would damage it or there are some important rules to follow in order to keep the board safe? I mean I would start from a vide start.S and add