An SD card is a solid-state device. This means it has no moving parts in order to function. This is a big improvement over older portable storage devices. Floppy disks, for example, had flimsy discs inside which spun at high speeds and were subject to data loss when placed near magnetic fields. CDs and DVDs are larger and less durable, and their storage capability degrades quickly with lots of rewrites. An SD card's components are part of its circuitry, and its packaging is small and durable compared to these predecessors.
Speaking of durable, we cracked open an SD card for the picture on this page. It wasn't easy, which is a testament to the aforementioned durability. The mini and micro sizes are more delicate, though, and you still don't want to store an SD card where it could possibly bend, chip or break.
Inside the SD card is circuitry that includes flash memory. In our article How Flash Memory Works, we cover how that circuitry controls the flow of electricity when writing or erasing data on the flash memory chip. Even though each SD card complies with standards for the form factor, capacity, speed and common input/output interface, different SD card manufacturers take different approaches in designing the flash memory and supporting components inside.
In compliance with standards, each SD card comes formatted with a File Allocation Table (FAT) file system, which is compatible with just about every operating system that might want to access the files in that system. Occasionally, you might need to reformat an SD card, either to "zero out" the data for a thorough wipe of all data, or to recover the file system after a corruption, which could happen when removing the card during critical read-write operations. When you need to reformat, the SD Association recommends using its SD Formatter software available here. One reason for this is that third-party disk formatters may not account for the Protected Area used in the SD card's CPRM functions.
So far, we've examined the SD card hardware and the technical side of how it works. Next, let's discover some of the challenges involved when using SD cards and card readers.