Christmas has come and gone, and many of you may have gotten a shiny new computer from Santa Claus this year. But before selling or giving away your old computer, you must prepare the hard drive for disposal. That means properly erasing all your files, to ensure that no confidential or sensitive information remains that is easily accessible.

Data is almost always at least partially recoverable. Recently an author bought a hard drive on eBay that had been used by a clinic. It had more than 30,000 credit card numbers on it. Drives purchased this way usually contain confidential data. Deleting your files or even reformatting the hard disk is not sufficient. Why?

Your hard disk has an index like the table of contents of a book. Say you do not want Chapter 6 anymore, and you delete it. Your PC just marks Chapt. 6 in the Table of Contents as empty. In other words, the area on the disk occupied by Chapt. 6 is now marked as available for other files.

But note that nothing else has happened. The actual data forming Chapt. 6 is still there, happy as a clam, in the same place on the disk. Much the same thing happens when you reformat your disk. In short, deleting files or reformatting the disk just removes the references to the data in question, not the data itself. This is why deleted files can be recovered with specialized software such as Disk Investigator. Data can even be recovered from reformatted disks. So what to do?

Before putting that old computer out with the trash or giving it away, go to one of these sites for a program that will wipe your individual files or entire hard drive. Note that some of these programs will erase Windows as well; in that case, of course, your old computer will be unusable.

There are many other similar programs available. Drop by your favorite search engine, and rummage around. These programs will ensure that your data is beyond recovery, for all practical purposes.

On a related note, you should also take precautions before sending your PC to the repair shop. Even if you trust the technicians with your stuff, your files may be deleted during the repairs. If your machine is working, back up all your important or sensitive files. Use a handy USB thumb drive or external hard drive, or one of many online (and free) storage services.

Personally, I prefer to use my own storage devices. This way, the data never leaves my hands. Whatever backup method you use, check that it successfully copied your files. Then delete those confidential files from your hard drive. You can reinstall them when you get your computer back from the shop.