Hard drive disappearing

[Tha General]

I have 500GB hard drive, down to 220 atm. Anyways, I notice sometimes it will read 190GB, but when I go to download something or whatever, or if a day or two pass, it goes back to 220GB. Do my computer needs to take hard drive space to do something? anyway to fix this?

I am picking up a second hard drive soon 1tb , but was curious about this issue .