Folder on unused drive takes 5 to 10 seconds to open, is this normal?

Truly had a hard time trying to describe a proper title.
Here's what I mean. Since I got this computer 3 years ago and along it Windows 7 I had this problem and questioned if it was a normal Win7 thing or not.
I turn on the PC and, for ex., I go online. This means I am presently working on my C: drive since that is where my OS sits and along it my Browser. But at one point, say 15 minutes later I want to open a file/folder which happens to be on a different drive, D:, E: whatever. Well at this point I hear coming from my computer, sitting below me, the drive begin to spin-and I need to wait from 5 to 10 seconds before I see anything on the screen--Windows Explorer. Is this normal or what? If not, what is going on exactly.
At this point, afterwards the folder siting on the drive takes no time to open, or a new folder or whatever but if I wait a while, say 30 minutes and then decide to open another folder sitting on another drive the same thing happens

Thank you very much it was quite informative. I guess it's a good thing whereby it's purpose for existence is to give the hard drive less wear-and-tear and creating less heat for the computer (and the drive itself).