Removing personal data from your old PC's disk

By Lisa Phifer

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One recent survey by found that seven out of 10 hard disks purchased through eBay still contained sensitive data generated by previous owner, despite having been “wiped clean” before resale.  Why? Because simply deleting files is not enough to eradicate the credit card numbers, passwords, bank statements and other personal data stored on your PC’s hard disk. So, when you sell or retire that old PC, how can you be certain that leftover data doesn't put you at risk?

Why DELETE doesn’t really mean ERASE
In many computer operating systems, including Microsoft Windows XP, deleting a file or folder does nothing to actually erase the data that it contains. In fact, your PC’s DELETE command is really more of a FORGET command.

Your PC keeps track of the files stored on your hard disk using a list of “file start” locations.  When you create a file, the data content of that file is actually spread across your disk: one chunk here, another chunk there, wherever your PC can find unused space.  When you read a file, your PC consults the list to find the right starting point and then gathers all of those chunks together so that it can present the email message or spreadsheet or Web page or whatever kind of information the file contains.

However, when you delete a file, your PC gets a bit lazy.  It just updates the list by over-writing the “file start” location and ear-marks those chunks of space as unused.  In essence, your PC just forgets about the file -- it will no longer be displayed by Windows Explorer or be accessible through ordinary applications like Word and Outlook.

As you continue using your PC, new data will gradually be written into that same space, replacing the older data.  In the meantime, plenty of information about your “deleted” file remains on your hard disk, for example,  file attributes like name and type or actual file content that was previously stored in all of those chunks (see Figure 1).

Figure 1: disk space summary statistics

The good and evil of file recovery
Dozens of file recovery programs take advantage of this deletion scheme. They can analyze your hard disk, looking for file names, words or phrases, and other interesting scraps of leftover data, attempting to recover (undelete) files that your computer has forgotten.

For companies that have accidentally lost mission-critical files through user error or system failure, these recovery programs are invaluable.  But the longer a disk continues to be used (or the more extensive the damage), the less successful file recovery becomes.  So, if the PC is shutdown right after a file is deleted, the odds are excellent that most, or even all, of that file can be recovered by even the most basic “undelete” programs.

Now, think about what happens when someone discards an old PC. They delete files, and then shut it down. Then the computer is resold or recycled. Those same recovery programs designed for good purposes can also be used by criminals to steal data.  From confidential business presentations and spreadsheets to email messages and Web pages that reflect online banking activities, your old hard disk is a treasure trove of valuable information.

Protecting yourself and your data
Fortunately, if you realize this before you sell donate, or discard your old PC, you can take steps to avoid becoming a data-theft victim.  Alternatives range from simple to complex, better-than-nothing to infallible, and harmless to destructive.  For example:

1. Over-write files on your hard disk.  After deleting all of your own files, you could just repeatedly copy a very large innocuous file as many times as it takes to fill your hard disk.  Doing so is easy.  Unfortunately, it is not terribly effective.  Some of your data will definitely remain and be recoverable with relatively little effort.

2. Securely delete files on your hard disk.  Alternatively, you could use a secure delete utility to thoroughly over-write designated files with random data.  These programs use automation and operating system interfaces to obliterate more of the information associated with your files.  For example, SDelete (a freely-available Windows utility) not only over-writes files, folders and partitions – but it over-writes the filename 26 times as well (see Figure 2).  It is easy to securely delete a few files or folders this way, but scrubbing an entire disk can take awhile.  Furthermore, some secure delete programs can be hampered by file compression, defragmentation or spyware.

Figure 2 (Sdelete)

3. Sanitize your entire hard disk.  To completely erase all user-accessible areas on a hard disk in one fell swoop, you could use a disk sanitizer like HDDerase.  This type of utility can rigorously scrub an entire disk with one command by using an industry-standard low-level interface built right into your hard disk.  The National Institute of Standards and Technology (NIST) Guidelines for Media Sanitization (SP 800-88) recommends HDDerase as a reliable non-destructive disk purge technique.  But to use HDDerase, you’ll need to jump through a few extra hoops: download this free utility, copy it onto a bootable floppy, CD or USB drive, reboot your PC from that external drive, then run the erase program (see Figure 3).

Figure 3. (HDDerase)

4. Physically purge your hard disk.  Because your PC’s hard disk is a form of magnetic storage, passing it through a very strong magnetic field can actually destroy the data recorded in fields distributed across the disk’s surface.  This can be accomplished by a special-purpose piece of hardware called a degausser (see Figure 4).  Recyclers use this method to clean video tapes and floppy disks because degaussers do their job instantly.  But unfortunately, degaussers also damage the magnets inside hard disks, turning them into high-tech doorstops.

Figure 4 (Deguasser)

5. Physically destroy your hard disk.  If you plan to damage your hard disk beyond repair anyway, then you’ll probably find physical destruction easier.  Remove the hard disk from your PC and any protective case, then carefully pulverize the disk itself with a sledge hammer.  Your goal should be to damage the disk and its connectors badly enough that nobody could connect it to another PC to recover your data.  Of course, destruction is a poor choice if you plan to resell or donate your PC, because the new owner will need to buy another disk.  But it is an easy and highly-reliable way to make sure that your data is lost for good.

Forewarned is forearmed
It’s important to avoid letting your data fall into the wrong hands when you get rid of an old PC.  But it is becoming increasingly important to avoid data theft throughout the PC’s life.  That can be accomplished through data encryption, a common technique used to scramble information so others who read it cannot interpret it.

In fact, if you encrypt your folders and files as they are written to hard disk, you won’t have to worry about destroying left-over data when you sell your PC.  File, folder and full-disk encryption are advanced techniques, beyond the scope of this article.  But most new PCs do have built-in file encryption capabilities for those who want to keep their data private at all times.  For example, Windows Vista PC users should check out BitLocker.  Those with Windows 2000 or XP PCs can use HELP to search for information on how to use the Encrypting File System (EFS). Mac users can look in System Preferences under Security and use FileVault.