Does a HDD ( Hard Disk Drive) emit radio waves? While I was reading the "guide" of a HDD I had bought a while ago, I found that it might emit radio waves. I was wondering if that is true and why that happens?
 A: Any changing voltage or changing current emits some EM radiation.  Electronic devices, such as a disk drive, contain digital circuitry that can change the voltage on various conductors inside quite rapidly.  The faster the voltage change, the more power will radiate, and the wider the spectrum of that radiation.
This is a well known issue for all electronic devices.  There is nothing inherently special about a disk drive, and you don't say anything about why you singled out that one piece of equipment.  In most jurisdictions, all devices sold must comply with certain maximum radiation regulations.  Here in the US, this falls under the jurisdition of the FCC.  See the FCC rules part 15.  I'm guessing you are asking this question because you saw a sticker on the unit saying something about it complying with radiation limits.
A hard disk drive is classified as a unintentional radiator by the FCC.  That defines emissions limits over a broad range of frequencies the device must stay below during normal operation.  You probably some something on the nameplate that states this compliance.
So yes, your disk drive will emit radio waves, just like everything else electronic you have.  However, the total power of those radio waves is tiny, and nothing to worry about.  The computer as a whole that the disk drive is in will radiate much more, but should in theoery still be under the same limits imposed on all unintentional radiators.  If your computer has a metal case, that case is probably integral to the EM suppression system.  In other words, if you run your computer with the case open, it may (and probably will) emit more radiation than the limit.  Even that though is of no concern to your body.
Here is something you can do for fun and might be instructive too.  Get a small portable AM radio and tune it between stations.  Now move it around near your computer when the computer is running.  You will hear all kinds of strange noises.  The noises might even change somewhat depending on what the computer is doing.  Now go do the same thing to other electronic devices.  If you think the computer was noisy, try holding the radio near your refigerator when its running, near your fuse box, or near your car with the hood up.  There is a lot of low power RF floating around, with disk drives being very very tiny sources in the scheme of things.
A: My external hard drive emits enough radio frequency to measure as comparable to an active  cell phone on an accousticom 2. Mine reached 1-3 v/m when placed several inches away. The reading was still high enough at 3 feet that I unplugged it. Not nearly so worrisome as your cell phone, but could still contribute to symptoms like headache in an electro hypersensitive individual. My computer is ethernet cabled (no wifi) and my tower does not emit a high RF (radiation) reading like that from the external HD.
A: It gives off magnetic and thermal radiation. As you might know, both are completely harmless, and the thermal radiation is only when it's turned on (it's also known as heat ;) 
On a side note, it's unlikely you'll find magnetics in a modern hard drive. The magnets are electromagnets and are very, very tiny. 
Blake Levitt points out in Electromagnetic Fields, it rests "literally on the user’s lap at genital level."(In case of Laptops)
