We are, it turns out, careless with our data. Researchers from BT and the University of Glamorgan (Wales) as well as "data wiping specialists" LifecycleServices (which I picture as a sort of digital mortuary) and the University of Edith Cowan in Australia bought and scanned some 300 used hard drives. According to the researchers, 49 percent (of those that worked) contained sensitive personal information. Two actually had material indicative of "potential" criminal activity; these were turned over to police.
This study has been done before with similar results. For better or worse, erasing your tracks is hard, even if you conscientiously try to do it. (I wonder how many people think dropping a file into the recycle bin is enough?) It's not like the good old days when you could use DOS's format command and be confident (couldn't you?) that your bits had gone to their reward. Format is still with us, though it's not available to the casual Windows user. (This is probably good: I'm sure it accounted for more than its fair share of suicides.) Online backup services further complicate things: The really effective ones will have copies of your "deleted" files safely ensconced in mountain bunkers, waiting patiently for a hacker or a subpoena. Our tracks have multiplied, too: In addition to the files we explicitly create, there are temp files and browser logs and registry entries.
An exciting new complication: Microsoft Vista apparently has a sort of "time machine" function that automatically retains old images of files. That feature could save your life, but it will probably make covering your tracks even trickier. As storage gets cheaper and people become more (offsite) backup-conscious (sysadmins pray fervently for this, so it'll surely happen someday), we can look forward to a time when information simply doesn't disappear, no matter how hard you might rub the eraser.