We've shown you
In other words, thanks to the way computers work, data is not gone for good without some extra work on your part. Hard drives and operating systems make you think data is deleted after you empty the trash, but it isn't. The space is just available for new files. Anyone with the right forensic tools—yes, think CSI if you want—can typically revive data you thought you were rid of.
Luckily, you can recover deleted data too. So when you accidentally delete a file, it doesn't have to be a disaster. But if you want that data truly destroyed, you have to shred it and over-write where it lived a few times, and then you can go on in peace, even if you sell the drive later. (In order to be sure that data can't be recovered, take that hard drive out back, drill holes in it, shoot it with grandpa's shotgun, smash it with a ball peen hammer, and set the remains on fire.)
Nuke It: Hard Drive Wipe
If you're going to pass an old hard drive on to someone else and you
want to make sure the data once on it is never seen again, you need to
security wipe it with a digital tool that will over-write it enough
times to make it impossible to recover the data. Just doing a fdisk
erase from the command line is not enough.
Overwriting usually comes in a couple of forms: the Gutmann method, which calls for overwriting data 35 times, and the U.S. Department of Defense's recommended method, which is seven overwrites. In addition, there's the Pseudorandom Number Generator (PRNG) Method, which uses random data in just four overwrites. Guess which one takes the longest? Doing 35 overwrites on a large-sized hard drive can take days. Literally. The DoD method and others should be fine. In fact, the Guttman method performs operations that might not even work with modern hard drives, so stick to a smaller number of over-writes.
Note, you probably don't have to go through this intense of a process if you're just going to reinstall the OS for yourself. A simple disk format should be fine.
A perfect—and free—tool for the job is Darik's Boot and Nuke (DBaN). You can download the software as an image and write it to a CD-ROM or USB flash drive using a tool like ImgBurn, and then boot your computer with DBaN as the temporary OS with one mission: keep writing over the disk until it's done. You can pick a quick erase, DoD method (three or seven passes), PRNG, Gutmann, even a data sanitization method created by the Royal Canadian Mounted Police that will do one to seven passes.
There's also a paid program that does the same thing: Remo Drive Wipe ($39). Pick a partition or entire drive and you get up to nine sanitization methods, from simple to a combo of DOD+Gutmann, including a standard used by NATO.
Shredders: Selective Data Wipes
There's no lack of file-shredding programs out there. What they promise
to do is make sure a file is actually deleted when you choose delete.
They overwrite the spot where the file(s) once lived on the hard drive,
enough that forensic software can't recover the data. Just be aware that
the over-write adds time to file deletion, so you may sense a lag when
you do a secure wipe.
Also, don't neglect the importance of free space scrubbing, where software looks at the supposedly empty space on your hard drive and scrubs it clean. If you are going to sell your computer, for example, you can format the drive, reinstall the OS, and then use the free space scrub to ensure your old files to ensure no one restores your old files.
MacOS: The Macintosh actually has a selection under the Finder menu called Secure Empty Trash, so you can do exactly that with whatever you dragged to the trash. You can also go into Finder Preferences to always Empty Trash Securely if you want. If you prefer a third-party tool, the free Permanent Eraser uses DoD or Guttman methods to permanently erase files.
Windows: The options are numerous. A few free options include Hard Disk Scrubber for both free space scrubs and selective shredding; Eraser, which can even run on a schedule you set; Easy Shred, which sits on the desktop for easy drag and drop deletion; and DeleteOnClick, so you can right click and delete a file right away (once the scrubbing is done).
If you simply want to wipe your empty disk space, one of our favorite tools for cleaning up the crap in Windows is Piriform's CCleaner, which now supports that very option. You can set this to happen automatically with each clean or manually run the Drive Wiper from the tools menu. As with any wipe, if you've got a big drive with lots of space, you'll be waiting awhile.
Wipe Online and Mobile Files
What about all the data you have in the cloud? What are the options for
permanent deletion of that data?
Take Google Docs, for example. Google says it's easy to delete a file: Select the item, choose Move to trash from the Actions menu, click the Trash, select Empty Trash at the top, and voila. But did you make sure the file was no longer shared? Did you, in fact, even own the document in question? You'll need to remove other owners and editors and viewers before you can even get as far as moving it to the trash. Even then, you can't count on it being gone. As far back as 2007, it was reported that Google Docs left an image of your document available on its servers—the very image it would make available via a URL for shared viewing.
Here's the smartest thing you can do: If your files are truly sensitive, DON'T PUT THEM IN THE CLOUD. Did you learn nothing from the congressman with the unfortunate last name? Sure, storing data with Google Docs or Windows Live SkyDrive is not exactly the same as putting pictures on Twitter, by any means, but all it takes is one unscrupulous person to intercept something you don't want to share. Then you've lost control of your own data.
What about data you have on your smartphone? Luckily, remote wipe options come for Apple iDevices and Android now. With any iPod touch, iPhone, or iPad, you get free access to the Find My iPhone app. Once you select your missing or stolen device, you can push the Remote Lock or Remote Wipe option. If the device is online, destruction starts immediately; if it's not online, it'll start happening the next time the device logs in. It's possible this advice could change slightly when iOS 5 debuts, but that's how it works now with iOS 4.
Android users can download the Android Lost application and set up an account. Sign into the Android Lost website to find a missing phone via GPS and then you can lock it or erase not only the phone data but also whatever is on the SD card you may have inside the phone. Windows Phone 7 users can use the Find My Phone function of their Windows Live account to find the phone on a map, lock it, or erase it. BlackBerry users are typically going to have to go to their network administrator and tell them the phone is lost or stolen to get them to do the remote wipe, but RIM does offer BlackBerry Protect app/service to consumer customers that should do the trick.