The Seagate external drives are extremely vulnerable to that clicking. We use several at my work and several of them show this behaviour, which is firstly scary and secondary very annoying if you're sitting next to the thing. I have always noted with concern how Seagate external drives claim to be 7200RPM, yet they cost less than the internal 7200RPM drives themselves do. So either they use dubious quality in these devices, or they aren't really 7200RPM.