The FreeAgent Pro drives come equipped with an amber light to indicate when power has been applied to the device.  When the drive is in use only a small portion of the amber light will pulse. The pulsing light is located in the front center of the drive.

See the image below for the correct location of the pulsing light:  

Never disconnect your FreeAgent device while the lights are pulsing on the front of the unit. Pulsing lights indicate that the drive is in use, so unplugging power or data cables while the drive is in use can cause data corruption or loss.