Hard Disk Drive (HDD)
Hard disk drives consist of one or more magnetically-sensitive platters, an actuator arm with a read/write head on it for each platter, and a motor to spin the platters and move the arms.
Each platter is organized into concentric circles called tracks. Tracks are divided into logical units called sectors. Each track and sector number results in a unique address that can be used to organize and locate data. Data is written to the nearest available area at random. There is an algorithm that processes the data before it’s written, allowing the firmware to detect and correct errors.
Reading and writing
Each time you ask your computer to retrieve or update data, the actuator arm physically moves the head to where that data is located, and the read/write head gathers the data by reading the presence or absence of a charge in each address. If the request was to update the data, the read/write head changes the charge on the affected track and sector.
The drawbacks to HDDs are a result of the mechanical parts used to read and write data, as physically finding and retrieving data takes more time than electronically finding and retrieving data. The mechanical parts can skip or even fail if they are handled roughly or dropped. HDDs are also heavier and use more power when compared to SSDs.
The benefits of a hard disk drivs are that they are a proven technology, and are frequently less expensive than a solid state drives for the same amount of storage. Currently, HDDs are also available with more storage space than SSDs.
SSD (Solid State Drive)
A Solid state drive uses flash memory to deliver superior performance and durability over HDDs since there are no moving parts inside your SSD unlike a HDD where there are magnetic heads, spindles, and spinning platters. Without moving parts, SSDs are more durable, run cooler and lower power consumption overall.
How NAND works
SSDs are essentially a large USB drive as they use the same type of technology that a USB drive uses. The NAND technology used in SSDs is a kind of flash memory. At the hardware level they work by floating gate transistors recording a charge or lack of charge to store your data. The floating gates are organized in a grid type pattern, which is then organized into a block. Block sizes can vary and each row that makes up the grid is called a page.
Reading and writing
Updating data is more complex for SSDs as all the data in a block must be refreshed when there is an update to take place. The data on the old block is copied to a different block, the original block is erased, and the original data with the updates are written a new block.
When the drive is idle, a process called garbage collection goes through and makes sure that any information in the old block is erased and that the block is free to be written to again.
There is another process, called TRIM, that informs the SSD that it can skip rewriting certain data when it erases blocks. There are a finite number of times any block can be rewritten, this is an important process that prevents premature wear on the storage drive.
To prevent premature wear on an SSD, there is an algorithm to ensure that each block gets an equal amount of read/write processes which is called wear levelling.
The read/write process requires data movement and there is a certain amount of space on the SSD that is not reported to the operating system, as well as not being accessible to you to allow the drive to move and delete items without affecting the overall storage capacity.
SSDs are newer technology and are more expensive than HDDs. It can be harder to find large-capacity SSDs as opposed to HDDs which can be as much as 2.5 times larger in capacity.
SSDs deliver faster load times because of the technology they use, they are lighter and better able to withstand damage from movement and being dropped. In addition, SSDs use less power, as well as allows your machine to run cooler.