SSD devices wear out faster if the same blocks are written and rewritten, so FTL balances the write load across all of the available memory. FTL figures out how to erase unused blocks of memory when the SSD isn't doing anything else. For example, SSD blocks have to be erased before they can be written, and erasing takes a lot of time. SSDs have to deal with the vagaries of Flash media, which are quite different from rotating magnetic layers. Operating systems want to work with file allocation tables and clusters. The culprit? SSD's so-called Flash Translation Layer, a firmware interface that makes an SSD appear to the PC like a big fat, uh, FAT device. If any of the original data remained, erasing didn't work. The study's method is straightforward: They put repeating data on an SSD or USB drive, tried using various erasing techniques, took the SSD or USB drive apart, and pulled raw data off the chips.

The tome, 'Reliably Erasing Data from Flash-Based Solid State Drives' (PDF), goes through all of the known techniques for erasing data and comes up short in every case. Researchers from the University of California at San Diego delivered a paper at the FAST-11 Conference in San Jose, Calif., last week that shows it's almost impossible to reliably erase data from a solid state drive.

How to erase a flash drive memory