3. Limitations of Flash Memory
One limitation of flash memory is that although it can be read or programmed a byte or a word at a time in a random access fashion, it must be erased a "block" at a time. Starting with a freshly erased block, any byte within that block can be programmed. However, once a byte has been programmed, it cannot be changed again until the entire block is erased. In other words, flash memory (specifically NOR flash) offers random-access read and programming operations, but cannot offer random-access rewrite or erase operations.
When compared to a hard disk drive, a further limitation is the fact that flash memory has a finite number of erase-write cycles so that care has to be taken when moving hard-drive based applications, such as operating systems, to flash-memory based devices such as CompactFlash. This effect is partially offset by some chip firmware or filesystem drivers by counting the writes and dynamically remapping the blocks in order to spread the write operations between the sectors, or by write verification and remapping to spare sectors in case of write failure.
The cost per byte of flash memory remains significantly higher than the corresponding cost of a hard disk drive, and that has prevented flash from becoming a solid state replacement for the hard disk drive on normal desktop and laptop computers.