Businesses and government offices are constantly replacing computers and buying new hardware.  Typically when this is done, data on the hard drives of the defunct machines is wiped, lest it fall into the wrong hands.

However, an intriguing study [press release] by researchers at the University of California San Diego (UCSD) reveals that businesses thinking they've wiped NAND thumb drives or NAND solid-state drives (SSDs) may be in for a surprise.

Every time you write to a hard drive -- be it magnetic disk or NAND -- you make semi-permanent changes that persist until you overwrite that block of memory.  When you delete files on your computer, you typically are merely deleting the indexes of the files.  The actual data persists on the drive until you overwrite it.

Over a dozen methods have been worked out to try to fully overwrite data on a magnetic hard drive and permanently erase any traces of the drive's original contents.  Researchers tried those methods on flash drives and discovered that, at best, they left 10 MB of every 100 MB file intact.

To study how successful the data destruction was, the researchers took apart an SSD.  Rather than check the Flash Translation Layer (FTL), which would merely show data as indexed by the drive, they actually sliced out the physical chips and queried them via their pins.  This allowed them to test the data status at the lowest level.

The findings might shock some, but came as little surprise to the researchers who expected magnetic drive techniques to work less than optimally for SSDs.  

Some of the techniques attempted, such as Gutman's 35-pass method, Schneier 7-pass method, erased as much as 90 percent of data successfully.  But other techniques, like using pseudorandom numbers to overwrite data on the chip or using a British HMG IS5 baseline, left virtually the entire file intact.

Researchers Laura Grupp and Michael Wei comment, "Our results show that naïvely applying techniques designed for sanitizing hard drives on SSDs, such as overwriting and using built-in secure erase commands is unreliable and sometimes results in all the data remaining intact. Furthermore, our results also show that sanitizing single files on an SSD is much more difficult than on a traditional hard drive."

Of course, if you encrypt all the data on the SSD to start, you make it harder to access.  The researchers note this and suggest that to completely prevent data loss, users then destroy their keys and use new technology to directly overwrite all of the drive's pages.

Chester Wisniewski, a senior security advisor for Sophos Canada, blogged on the study praising its accuracy.  He writes, "To properly secure data and take advantage of the performance benefits that SSDs offer, you should always encrypt the entire disk and do so as soon as the operating system is installed... [S]ecurely erasing SSDs after they have been used unencrypted is very difficult, and may be impossible in some cases."

These results are not only troubling for business and government users, but for home users as well.  You have plenty of things to worry about falling into the wrong hands -- personal emails from your family; credit card records; medical records; and other private info.  At present, you can't be 100 percent sure you can securely dispose of SSDs with this kind of information, but by using encryption you can reduce the likelihood of someone get your information to almost zero. 

According to a recent iSuppli report, only 2 percent of laptops currently carry SSDs.  However, iSuppli predicts that by 2014, that total will rise to 8 percent.