I have a pretty beefy iMac. 3.1 GHz quad core i5, 16GB memory and a factory SSD. Attached to this drive is a spinning 2TB HDD which I have partitioned to be have a drive for time machine and another for my iTunes library. Now with the power of the machine, you would think that it would be pretty snappy, and it is in general, but occasionally, the external drive will be asleep, meaning not spinning, and I will do something that in theory, shouldn't be accessing the spinning drive. However, it an app will pause while I listen to the drive spin up and only become response once the drive has reached speed. Like I said, only my iTunes folder and a time machine backup are on the drive, but it has been triggered by all kinds of things. For instance, just now I had a text edit document open, never saved, and I start to type into it and I get the pause. Other times I will be browsing the SSD and it will pause to fire up the drive. It happens all the time with no reasonable connection to the drive.