Memory sticks, SD cards and solid state drives do not have moving parts with a spindle going around and a header reading and writing data from them, because of this, their power consumption is very small and data access much quicker than it is on a traditional drive, albeit they are also more expensive.
Solid state disks are nothing else than a bunch of thumbdrives bundled together in one piece, they work the same way a small thumb drive memory stick would do but with bigger capacity to store data, the technology they use to read and access data it is the same and they share the same problems when it comes to securely erase data from them.
Internal working of storage devices using flash memory
Memory thumb drives and solid state disks contain a chip that can be electrically erased and reprogrammed, hence the lack of moving parts.
Modern flash memory chips are divided into segments called blocks or sectors, to erase a flash cell a large voltage of the opposite polarity is applied, the problem is that your thumb drive flash memory has a finite writing and erasing cycling capability. The repeated use of the same number of blocks in flash memory can cause the solid state disk, USB memory stick or SD card to wear out and become corrupt or stop working.
Wear levelling technology explained
To preserve the life of your memory stick or SSD disk, flash based drives use a technique called ‘wear levelling’. Wear levelling technology will shift data around the physical blocks of the flash drive preventing the same physical block being used over and over again.
Let’s imagine you have a 32GB solid state disk, SD card or memory stick in which resides 100MB of personal photographs you want to securely erase beyond recovery, if you use a wiping utility to overwrite the data, wear levelling technology integrated within the file system will track how many times each block is used will take the free space from somewhere else in the disk regardless of the address of the old block, any old data will just deleted and not overwritten paving the way for its easy recovery with specialist software.
How many times should I overwrite data on a memory stick to make it irrecoverable?
ONE single pass of zeroes or random data is enough to erase data beyond recovery on a flash memory stick, SD/MMC card or solid state disk.
Overwriting data on a flash drive more than once only wastes time and write cycles, the only thing you need to make sure of is that the whole of the memory thumb drive or solid state disk is completely overwritten, in other words, you can not safely erase a single file from a memory stick but you can safely erase the whole device and by doing this you will be making sure that all blocks are overwritten and not shifted to other logical addresses in the flash memory.
Tips to securely erase data on USB memory sticks and solid state disks
- Securely wiping an individual file on a flash drive is not possible since the random data that is used to overwrite could be written off to a different physical block.
- You will need to overwrite all of the flash memory to make sure that random data used to overwrite a file is spread evenly everywhere without skipping any memory sectors.
- A single pass is enough to securely erase data on a memory stick, the secret for securely erasing data on flash memory lies on making sure everything is overwritten and not in the number of passes used.
Before overwriting data on a USB thumbdrive remember that some of them have a little switch that you have to physically move to turn on/off write protection.