Solid State Drive (SSD):

 

A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is also sometimes called a solid-state device or a solid-state disk, even though SSDs lack the physical spinning disks and movable read–write heads used in hard disk drives (HDDs) and floppy disks.

Compared with electromechanical drives, SSDs are typically more resistant to physical shock, run silently, and have quicker access time and lower latency. SSDs store data in semiconductor cells. Hybrid drives or solid-state hybrid drives (SSHDs), such as Apple's Fusion Drive, combine features of SSDs and HDDs in the same unit using both flash memory and a HDD in order to improve the performance of frequently-accessed data.

SSDs based on NAND Flash will slowly leak charge over time if left for long periods without power. This causes worn-out drives (that have exceeded their endurance rating) to start losing data typically after one year (if stored at 30 °C) to two years (at 25 °C) in storage; for new drives it takes longer. Therefore, SSDs are not suitable for archival storage. 3D XPoint is a possible exception to this rule; it is a relatively new technology with unknown long-term data-retention characteristics.

SSDs can use traditional HDD interfaces and form factors, or newer interfaces and form factors that exploit specific advantages of the flash memory in SSDs. Traditional interfaces (e.g. SATA and SAS) and standard HDD form factors allow such SSDs to be used as drop-in replacements for HDDs in computers and other devices. SSDs have a limited lifetime number of writes, and also slow down as they reach their full storage capacity.

 

Comparison of NAND-based SSD and HDD
Attribute or characteristic	Solid-state drive (SSD)	Hard disk drive (HDD)
Price per capacity	SSDs generally are more expensive than HDDs.	They are cheaper than SSDs.
Storage capacity	In 2018, SSDs were available in sizes up to 100 TB, but less costly; 120 to 512 GB models were more common.	In 2018, HDDs of up to 16 TB were available.
Reliability – data retention	If left without power, worn out SSDs typically start to lose data after about one to two years in storage, depending on temperature. New drives are supposed to retain data for about ten years.	If kept in a dry environment at low temperatures, HDDs can retain their data for a very long period of time even without power. However, the mechanical parts tend to become clotted over time and the drive fails to spin up after a few years in storage.
Reliability – longevity	SSDs have no moving parts to fail mechanically so in theory, should be more reliable than HDDs. However, in practice this is unclear. SSDs have undergone many revisions that have made them more reliable and long lasting. New SSDs in the market today use power loss protection circuits, wear leveling techniques and thermal throttling to ensure longevity.	HDDs have moving parts, and are subject to potential mechanical failures from the resulting wear and tear so in theory, should be less reliable than SSDs. However, in practice this is unclear. When stored offline (unpowered on the shelf) in long term, the magnetic medium of HDD retains data significantly longer than flash memory used in SSDs.
Start-up time	Almost instantaneous; no mechanical components to prepare. May need a few milliseconds to come out of an automatic power-saving mode.	Drive spin-up may take several seconds.
Sequential access performance	In consumer products the maximum transfer rate typically ranges from about 200 MB/s to 3500 MB/s, depending on the drive. Enterprise SSDs can have multi-gigabyte per second throughput.	Once the head is positioned, when reading or writing a continuous track, a modern HDD can transfer data at about 200 MB/s. Data transfer rate depends also upon rotational speed, which can range from 3,600 to 15,000 rpm and also upon the track (reading from the outer tracks is faster). Data transfer speed can be up to 480 MB/s (experimental).
Random access performance	Random access time typically under 0.1 ms. As data can be retrieved directly from various locations of the flash memory; access time is usually not a big performance bottleneck.	Read latency time is much higher than SSDs. Random access time ranges from 2.9 (high end server drive) to 12 ms (laptop HDD) due to the need to move the heads and wait for the data to rotate under the magnetic head.
Noise (acoustic)	SSDs have no moving parts and therefore are silent, although, on some SSDs, high pitch noise from the high voltage generator (for erasing blocks) may occur.	HDDs have moving parts (heads, actuator, and spindle motor) and make characteristic sounds of whirring and clicking; noise levels vary depending on the RPM, but can be significant. Laptop hard drives are relatively quiet.
Susceptibility to environmental factors	No moving parts, very resistant to shock, vibration, movement, and contamination.	Heads flying above rapidly rotating platters are susceptible to shock, vibration, movement, and contamination which could damage the medium.
Installation and mounting	Not sensitive to orientation, vibration, or shock. Usually no exposed circuitry. Circuitry may be exposed in a card form device and it must not be short-circuited by conductive materials.	Circuitry may be exposed, and it must not be short-circuited by conductive materials (such as the metal chassis of a computer). Should be mounted to protect against vibration and shock. Some HDDs should not be installed in a tilted position.
Susceptibility to magnetic fields	Low impact on flash memory, but an electromagnetic pulse will damage any electrical system, especially integrated circuits.	In general, magnets or magnetic surges may result in data corruption or mechanical damage to the drive internals. Drive's metal case provides a low level of shielding to the magnetic platters.
Weight and size	SSDs, essentially semiconductor memory devices mounted on a circuit board, are small and lightweight. High performance models often have heat sinks attached to the device, or have bulky cases that serve as its heat sink, increasing its weight.	HDDs are generally heavier than SSDs, as the enclosures are made mostly of metal, and they contain heavy objects such as motors and large magnets. 3.5-inch drives typically weigh around 700 grams.
Read/write performance symmetry	Less expensive SSDs typically have write speeds significantly lower than their read speeds. Higher performing SSDs have similar read and write speeds.	HDDs generally have slightly longer (worse) seek times for writing than for reading.
Power consumption	High performance flash-based SSDs generally require half to a third of the power of HDDs. High-performance DRAM SSDs generally require as much power as HDDs, and must be connected to power even when the rest of the system is shut down. Emerging technologies like DevSlp can minimize power requirements of idle drives.	The lowest-power HDDs (1.8-inch size) can use as little as 0.35 watts when idle. 2.5-inch drives typically use 2 to 5 watts. The highest-performance 3.5-inch drives can use up to about 20 watts.
Maximum areal storage density (Terabits per square inch)	2.8	1.2