More recently introduced terms such as 'annually laminated' are synonymous with varve. Of the many rhythmites in the geological record, varves are one of the most important and illuminating in studies of past climate change. Varves are amongst the smallest-scale events recognised in stratigraphy.
PCs are designed to shut down automatically when heat endangers the hardware. To find out if this is your problem, download and install Speedfan. It's small and free. Although Speedfan's basic job is controlling the speed of your PC's fans (you probably guessed that by the name), it also provides the CPU temperature.
And it certainly only applies while the computer is running. If the computer's powered down, it has no way to tell the external drives it's time to shut down now. Even with the power on, the drives themselves automatically turn off after some period of inactivity.. That's why I say it kind of depends on what your external drives do when they get bored, as I call it.