I need to wipe SD card so that information from it is not recoverable. If I perform full format in Windows by unchecking the "Quick Format" check box and format SD card that way is the information still recoverable?
1 Answer
Sort of. It wouldn't be easily recoverable, because the storage controller would report that nearly every sector contains only null bytes. However, due to the way that NAND flash memory works, all SD cards (and other memory cards, and SSDs, and so on) contain rather more storage than they advertise, and perform "wear leveling". This is needed because a given NAND flash memory cell can only be overwritten a limited number of times, so if there was a constant mapping of logical addresses (what the computer sees) to physical addresses (actual memory cells), any location that got written to a lot (for example, a file being frequently edited) would quickly wear out. Thus, flash storage devices have a controller that dynamically re-maps logical addresses to a new physical address - one that hasn't been written to much - when you write to it.
The consequence is that there is no way, from the perspective of the OS, to reliably overwrite an entire flash storage device. In fact, the locations that you most recently wrote to will be among the last to get overwritten again. You could overwrite the whole card multiple times, in hope that the wear leveling algorithm will eventually clear all the physical cells (at the cost of reducing the card's usable life), but that isn't reliable. The number of times a memory cell can be written to is not constant, and some may fail early, in which case the controller will mark that entire block unwritable and never again overwrite it (this is another reason flash storage devices contain storage more than their advertised capacity, so that failed blocks can be removed without affecting capacity). However, failed blocks are often still mostly or entirely readable.
So, as in so many questions here, you need to consider your threat model.
- If you are only worried that somebody might find the card, put it in their PC, and see files in Explorer/Finder/etc., then you don't need anything but a quick format.
- If you are worried that somebody will use file recovery software - which looks for files that were removed from the file system, but not actually overwritten - then you need to do a full format.
- If you are worried that somebody will use (expensive) data recovery hardware - which can bypass the controller and access the physical storage directly - then you should first fully format and then physically damage the card such as by cutting it apart; full overwriting is impossible to guarantee.
- If you are worried that somebody will use very rare and expensive data forensics hardware and techniques, which can attempt to recover data even from broken shards of silicon, you need to physically destroy the card (e.g. get it hot enough to fully melt the silicon), and also probably shouldn't be revealing on an Internet forum that you have enormously valuable data stored on SD cards.
