Prentke Romich Company
http://www.prentrom.com/support/article/1716
On the top - middle of the ECO case, you will find 3 lights.
The first light is a lightening bolt which will blink when the charger is plugged in to let you know the unit is taking a charge. If it is blinking rapidly, the batteries are in the charging process. If the charging light is blinking slowly, this means the batteries are fully charged although are trickle charging to maintain their full status.
The middle light is the Low Battery Light. This lets you know that the batteries are very low and the device is about to turn off unless the charger is plugged in.
The last light on the right is the Hard Drive Light. This lets you know when the Hard Drive is running or that the ECO is in the process of running software.
IMPORTANT NOTE: You should NEVER reset the ECO while the Hard Drive light is blinking.