This morning I discovered the drive's outer and inner rings were flashing one after the other, and both drives made a clicking sound. When I unplugged and plugged on again, both rings will flash on only once and both hard drives are making the clicking sound. I attached the drives to a PC using the power cable only and both seem to start up fine (with the usual bootup spin sound).
Could this be caused by a faulty AC adaptor or circuit board? This drive was on continuously for 1 1/2 years.