When you delete any kind of data from a storage unit, in reality what you're looking at is at what I call "access removal". In reality, bits and other pieces of that data remain in that unit. You can even format a drive and still there are ways to get to that information. Bottom line is that if someone wants to get to it, they can, regardless of the OS. Hence you have to ask yourself: How important is the data stored in my old computer?
I'm amazed at how often people get rid of old systems and either simply hit delete on most of their files or worst don't even do that leaving all of that available for anybody to steal. There are several simple ways you can wipe the storage unit in your old computer before you dump it somewhere else. As always we're keeping it simple, hence going straight to a pair that does the trick and just works:
1. Piriform's CCleaner (http://www.piriform.com/ccleaner) - One of my favorite software tools is CCleaner. It's nothing short of one of the best tools you can get for either Windows PC's or Mac PC's. and literally it's free. It helps you to protect your privacy online and makes your computer faster and more secure allowing you to perform data cleanups in multiple ways. Easy to use and a small, fast download, and did I mentioned free! In the tools area it has the "Drive Wiper" tool. Use carefully as this gives you the option you doing a whole drive wipe with different levels of security such as an NSA level wipe as shown in Figure 1.
|Figure 1: Piriform's CCleaner Drive Wiper utility will allow you to wipe any storage unit and give you multiple levels of wipe and other options making it an easy, GUI based tool for security and maintenance.|
The only drawback is that it can't delete the drive you're running you OS, but and easy work around is to simply, delete your data and then use the Drive Wiper to Wipe the Free Space Only in that drive. That will pretty much assure your data is gone after been deleted and the good thing is that if you want to pass or donate your computer, you can leave it operational for someone else to use.
2. Darik's Boot and Nuke (http://www.dban.org/) - This is a disk erasure tool whose main purpose is simple: Boot your system and erase the hard drive. This is a more dramatic approach as the name suggest, the idea is to nuke your hard disk leaving your computer unable to boot. Pretty much for anybody to re-use it, they'll have to do a full install and setup of an OS in that computer to be able to use it again.
Both of these offer different approaches, but the end results is the same: Getting rid of the data. As a bonus, both are freeware or "donate for use" so be thankful if possible to keep them going.
After these is the data absolutely gone?
Well the general assumption is that within reason yes. After all these are software based solutions. If only software is intended then use full hard drive encryption to make it even harder to recover any kind of usable data. Finally, if your data is really that important, the true way of making sure there's not way to access it is to physically destroy the storage unit. From shocking it with high voltage, a huge electromagnet or simply sending it to a hard disk shredding company is the ultimate insurance...