I have heard this one so many times that I just had to write about it: people complain that they bought a 320GB HD and it’s only reading 298GB. Or they just bought a 750GB HD and it’s only reading 693GB. Or… the list goes on and on.
I’ve also heard people purchasing RAM upgrades and then complaining the system is not recognizing the installed RAM.

Hard drives are measured in the 1000bytes = 1KB, 1000KB = 1MB, 1000MB = 1GB, 1000GB = 1TB etc by the manufacturer. The computer, however, will read 1024bytes = 1KB, 1024KB = 1MB, 1024MB = 1GB etc. You can see there’s a difference. The larger the drive, the larger the difference. A “40GB” drive should show up as roughly 37GB for a loss of 3GB. A “1.5TB” drive should show up as 1.38TB, for a loss of 114GB. Generally we just tell people this is due to formatting. It’s easier to explain that way.

RAM is measured in the computer-used 1024 base scale. There could be a number of reasons why the RAM is not showing up correctly. I’ll list some reasons, starting with the most common:

You have Shared Video Memory – A small amount of your physical RAM is actually used to run the video of the computer (anywhere from 1MB to 256MB)
You’re running a 32bit Operating System – 32bit OSes can generally only read up to 3.25-3.5GB of RAM. There are a few exceptions, but this is a general rule of thumb – so even if you install 16GB, the system will only read the first 3.25-3.5GB.
Your board can not read the chip you installed – This usually happens on older computers. If your board only supports 256MB chips and you install 512MB chips, the board will either not read the installed RAM or only read the first 256MB. A BIOS update can alleviate this issue.

For the most part that’s all there is to it.

Oh another part:
RAM is a volatile storage device that allows instructions to be stored randomly for use by the CPU. When you turn off your computer, you will lose whatever was in the RAM. RAM can transfer data at sustained multi-GB/sec speeds.
Hard Drives (HD) is a physical media storage device that allows users to store data permanently. When you turn off your computer, you will not lose the data on your HD (hopefully anyway). HD’s can transfer data at sustained maximum of 90MB/sec. Bursts are generally limited to less than 120MB/sec. That is as fast as the physical drive can go – most users will experience system performance within the 20-40MB/sec range.
RAM is faster than the HD.