I was talking to a friend of mine last night that I used to work with years ago and he sounded depressed. I asked him what was going on, he began with "I have this 2TB drive..." That's when I stopped him. I knew what he was going to say next.
So, again, if you don't want to lose your data, back it up before you park it on your local hard drive. There are lots of paid online services that do this for you automatically if you're not disciplined to do this on your own. I will provide a link on this site to one that's mac-centric in the coming days.
If you are disciplined, back up data to a separate drive, or to a CD or DVD. Don't trust your data in one place and don't assume that your hard dirve is safe because it's not.
Erik