Did you know that when you delete a file from your hard drive, it doesn't really go away? Did you know that even formatting a hard drive won't do the trick? It's very true.
When files are erased (and I use that term loosely) from a hard drive, they're not really gone, only the location information for the file is removed. In other words, the file(s) are invisible to the operating system (like Windows or Linux) but not impossible to recover.
So what do you do if you're selling or giving away your hard drive? Do more than just format it, that's for sure. What you need to do is completely erase the data in a way that makes it impossible (with current and probably any technology) to recover.
You can do that with special software called data destruction software. I keep a list of free ones here: Free Data Destruction Software. Basically you boot to a disc you burn and then follow the directions given by the software.
Here's a general walk-through of the process: How To Wipe a Hard Drive.
Did you properly wipe the last hard drive you passed on or disposed of?