Can data stored on an SSD be secured?

Study finds the task to be very difficult; overwriting or crypto-erasure seem the best methods for sanitizing SSDs

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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.

"And some drives don't erase all that data," said Gregory Wong, an analyst with market research firm Forward Insights.

For example, on most of today's SSDs wear-leveling algorithms are used to more evenly distribute data across the drive so as to not wear out any one area of the NAND flash. The problem is, wear leveling can also defeat data erasure because it relocates blocks between the time when they are first written and then overwritten.

The National Institute of Standards and Technology (NIST) is currently being pushed by the SSD industry to redefine some of the military erase overwrite protocols to recognize encrypting drives that can be cryptographically erased without the need to overwrite the flash.

"But that's not happening tomorrow. Government agencies take a long time to embrace standards," Smith said.

Lucas Mearian covers storage, disaster recovery and business continuity, financial services infrastructure and health care IT for Computerworld. Follow Lucas on Twitter at @lucasmearian, or subscribe to Lucas's RSS feed . His e-mail address is

Copyright © 2011 IDG Communications, Inc.

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