An SSD's FTL performs the mapping of data between the logical block addresses (LBAs) via the ATA or SCSI interface and NAND flash memory's physical pages.
In a paper titled " Reliably Erasing Data from Flash-Based Solid State Drives", the university researchers wrote that "all single-file overwrite sanitization protocols failed: between 4% and 75% of the files' contents remained on the SATA SSDs."
USB flash drives didn't fare much better. Between 0.57% and 84.9% of the data remained on the drive after an overwrite was attempted.
The researchers even attempted overwriting free space on the drives and defragmenting the drive to redistribute data, encouraging the FTL to reuse more physical storage locations, but it proved to be ineffective.
Of 12 SSDs they tested using the drives' native "Erase Unit" command, only four were actually erased. One SSD had reported itself to be sanitized, yet the data was recoverable by the researchers.
In a separate overwriting test, which took up to 58 hours some of the SSDs, researchers found one out of eight remaining disks came back as sanitized. After two overwrites, all but one came back as erased. One drive still had 1% of its data even after 20 overwrites.
Sanitizing a hard disk drive is a simpler task, the researchers found. At the consumer level, hard disks can be reformatted and overwritten. For commercial users, a degausser, which uses a strong magnetic field to demagnetize the disk platters, can effectively erase all data.
But SSDs don't function in the same way as HDDs.
On a hard drive, the write and erase sectors are the same, meaning when a host overwrites data, it goes to the same block as the original data had been written to.
Flash memory is made up of pages and blocks. Data is written in 8KB pages, and erase operations occur in 2MB blocks, also known as "chunks." Therefore, when an erasure occurs, an entire 2MB block must be marked for deletion.
So, when data is written to NAND flash memory it's a two-step process known as a read-modify-erase-write cycle. First, existing data must be erased and then the old data combined with the new can be written to a different page on the memory. The old data, however, isn't actually erased at the time of a new write; it's only marked for deletion.
Manufacturers use 'garbage collection' algorithms to go back at a later time, typically when a drive is idle, and erase data marked for deletion. All NAND flash devices work this way. In the meantime, duplicate data exists on the NAND flash memory.
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