Mass Storage Basics - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 11

Erasing
The media in a hard drive can be erased and rewritten virtually endlessly,
while flash memory can survive 10,000 or more erase cycles, depending on
the technology. Some memory cards contain programmed ROM chips,
which can't be erased and rewritten.
The flash memory used in storage devices must be read and written in pages
and erased in blocks. The page size for read and programming (write) opera-
tions is typically either 528 bytes (small block) or 2112 bytes (large block).
A 528-byte page can hold one 512-byte sector and 16 additional bytes for
error-correcting codes (ECC), address-mapping information for use in wear
leveling, and other information. A 2112-byte page holds four 512-byte sec-
tors with 16 additional bytes per sector. Newer memory chips tend to use
large blocks.
The block size for erase operations is much larger than the page size for read-
ing and writing. In the past, blocks of 16 KB and 32 KB were common,
while current flash memory has erase blocks of 128 KB or 256 KB. Before
writing to previously programmed memory, the area to be written must be
erased. So to write even a single byte to a previously programmed area, the
memory controller must erase an entire erase block and then program a
page's contents back into the memory.
The controllers in flash-memory cards manage the erase operations and
enable device firmware to work with 512-byte blocks. To write a byte to a
flash-memory card, device firmware typically reads 512 bytes into a buffer,
changes the byte to be written, and writes the buffer back to the memory
card. The card's controller handles the erase and write operations.
The controllers in flash-memory cards use wear-leveling techniques that
help extend the useful life of the memory array by spreading erase/write
cycles evenly among all of the memory cells. A file-system driver that
accesses raw flash-memory chips can implement wear leveling as well.
Security
Some media types have built-in copy-protection capabilities. For example,
an SD Card can be configured to require authentication before allowing
access to the card's contents, and a card can restrict the number of allowed
copies.