Greetings, folks!
In order to install Ångström refer to
http://beagleboard.org/Getting%20Started#update
The tricky part is the one mentioned in Step #8.
Make sure that the board is completely shut down and free of unneccessary influences like USB-devices, screens connected via HDMI, network cables,...
According to the manual it should not make a difference either to supply the device via USB or a powerplug. However, I would suggest using only the powerplug, because apart from the fact that the operating system (Windows in my case) continuously tries to connect I got the impression that it would somehow interfere with the installation.
Best,
MrJordn