USB Flash - Built in failure

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With the slow decline in CD's and the long lost days of floppy diskettes, USB portable storage has become common place. A memory stick, thumb drive or pen drive are common terms for the same thing, a USB mass storage device based on FLASH, and yet many people don't know that the whole technology behind FLASH storage has a very limited lifespan - this leads me on to the relatively high volume of data recovery requests we have for USB storage coming through the channel.

Flash memory is generally of two types, NAND and NOR. Both technologies allow permenant storage of data without needing a power supply. NAND requires data to be read and written in blocks called 'pages' and is by far the most common FLASH memory in use today.

FLASH memory like all memory stores data in 0's and 1's in a vast array of cells, but the method by which the data is permanently written involves pushing a charge (electrons) through an insulated layer, once through the insulator its stuck there and will remain until its pulled back through the insulator therefore changing the state.

However, this 'pushing' and 'pulling' through the insulator, known as tunnelling slowly breaks down the insulator until it fails. When an insulator fails this only effects the cell, but of course just one bit that won't switch will adversely effect the data when read back. Furthermore certain areas of the flash drive are read and written much more than other area's and these are the master directory and the File allocation tables, both of which are changed when data is read (changing last access time) and written (changing last updated time and changing allocation of storage in the file allocation table). This means that in many instances the part of the flash drive that fails first is the most important part - the part that tells us what files are stored on the drive and where they are stored.

Cheap vs Expensive

When it comes to Flash Drives, there is a real physical difference between the budget end of the market and the professional end because NAND/NOR Flash comes in many different flavours depending on its performance and expected lifespan. Often the cheapest FLASH IC's are designed for storing firmware in embedded devices where write performance is a non issue and the expected number of writes is very limited, maybe 10 writes in its entire lifetime whereas the most expensive FLASH is designed specifically for high speed  and many write cycles and this is the correct hardware for USB Flash Drives. If you can buy a 128GB Flash drive from SANDISK for £30 and a unbranded one for £5 then the lifespan and performance of your SANDISK drive will be many many times better than the unbranded one.

I guess I should also point out that some cheap unbranded USB Flash drives (or knock off Branded) are engineered to falsely report their capacity. This is done by creating a partition on the drive with false data, so the computer you connect it to thinks its larger than it is and the only way to be sure is to try and fill it up or to perform a low level reformat. This sort of storage fraud is often seen on sites like eBay promising 1TB of flash for $10 which is nonsense.

Recovering data from failed Flash drives isn't that hard, but it does bring with it some challenges because the data will have errors in it where specific cells are stuck or indeed entire pages are stuck and non responsive and its not always possible to identify these area's during the scan, they often read as ok but with incorrect data, or they read as all 0's but after re-assembling the filesystem as best we can its over to the client to work through the recovered data and validate it.

The bottom line here is never ever rely on a USB Flash drive for data storage, its not safe and certainly not guaranteed and it will fail at some point. Stick with brand names and stay away from the budget end of the market.

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Sunday, 17 December 2017

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