Raspberry Pi Status LEDs
D – I could not find any detailed description for the status LEDs that are on the Pi board next to the USB ports. While plowing through a few resources and thinking about it a bit more I now know what the status LEDs indicate. The list below starts with the outermost LED.
100M – yellow – indicates if the network connection is 100 Mbit or not – if it is lit up the connection is 100 M, if it is off the connection is 10 M
LNK – green – indicates that a physical network connection exists – if it is lit up the network connection is OK, if it is off the cable is unplugged on either end or broken
FDX – green – indicates that the network connection is using full duplex – if it is lit up the connection is full duplex, if it is off the connection is half-duplex
PWR – red – indicates that the Pi has power – if it is lit the power is connected, if it is off the power is not connected or the power supply is unplugged / broken
ACT – green – indicates SD card access – if it is lit the SD card is read from or written to, if it is off then there is no SD card access of any kind
Addition: I came across this post http://elinux.org/R-Pi_Troubleshooting#Green_LED_blinks_in_a_specific_pattern . What is a bit odd is that the color description of the LEDs is different in that post. On our Pis there are three green LEDs. I have yet to confirm which one of them is the one that reports any errors, but if I had to guess I’d guess it is the ACT LED.
It is now confirmed that the error reporting is done through the ACT LED. Makes sense as the other green LEDs are controlled by the network adapter that is attached to the onboard USB hub and therefore cannot be controlled before the OS was loaded.