Hi I am curious. I have an early 2008 24" iMac, the internal drive is gone. Luckily I have a clone of the internal drive on an external USB drive, I am using the USB drive as the start up drive, it works well. But I keep hearing clicking noise from the USB drive when the iMac is not in use(screen is off from the power saving mode. but the iMac is running.) I am wondering why the iMac keeps accessing the USB drive? My iMac has OSX 10.9.2. Thanks in advance.