The HDD light indicates that the Aurender internal hard drive is spinning. When you add songs to the play queue, the Aurender internal hard drive(s) will "spin up" to load the selected files to the solid state cache for playback(Only when the song selected cannot be found on SSD cache, which means never the song was played before). As soon as the caching is complete, (after about 5min's wait to see the new songs needed to be added from the HDD)the hard drive(s) will spin back down. When the drive(s) return to their idle state, the HDD light will disappear.