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Would anyone be able to explain why a 64GB Solid State Drive is significantly larger physically than a 64GB SD card? What are the main differences between the two technologies that lead to the size difference?
First, that 64GB SD card is much newer than the several year old 64GB SSD. Second SD cards use the much simpler & slower SPI type interface, SSDs have sophisticated SATA controllers often with cache DRAM and extra Flash memory bad blocks can be mapped out to.
SD card usually contain a single memory element. You write to that single element sequentally. You also wait for that device to complete a write before the next block can be written.
SSDs contain several memory elements (call it n in total) that can be written at once and sequentally across all of them and once the nth element is written, you have time to start again at the zero element which should have finished writing thus making writing much faster.
e.g writing 8 blocks sequentailly to a single element will involve 8 waits on an SD card. Doing the same that has 8 parallel elements allows you to write to all of them and wait once.
The main point is that the SSD consists of a bunch of memory chips, lets say 16 chips, and this effectively increases the throughput of the disk. Another factor is that larger area allows for better cooling during peak performance, and also the fact that disks used to come mainly in 3.5" and 2.5" form factors, so the manufacturers keep the 2.5" form for compatibility with other stuff. You can get SSDs in smaller packages but they will either cost a lot more or won´t be as fast as the normal ones.