Unless it gone bad, your hd will be running the entire time you have your pc on. I think you were meaning to say that the activity light never came on, or that you never heard anything being written to disk. I suppose some hibernation modes might allow the hd to go into a sleep state in a laptop, but I'm not familiar with that functionality / operation. Of course now you have solid state drives without moving parts, just dense populations of non-volatile memory, ie. it doesnt lose its data like RAM when its powered off. No noise, bearings, read/write heads, or spinning platters.
On a side note, in the 70s when I worked for a mainframe shop, it took an entire room of spinning platter clusters on elevated air conditioned floors to provide as much storage as you find on a telephones sd card. Memory was expensive, thus programs were written to use as little memory as possible, thus the Y2K problems. All years were two digits, assuming the starting point was 1900. When the century turned over, 2001 would look like 1901, etc.
One more thing, isnt a ram drive generated in 2mg chunks on the systems hard drive? I might be thinking of virtual memory, either way, it is released after use and/or power is cycled on the pc. Nothing retained.