Storage device is a hardware component that is used to store the operating system, applications and data on a computer. The data may be stored temporarily or permanently. Consequently, storage devices are classified into two categories, primary storage and secondary storage.
Primary storage devices store data temporarily and are internal to the computer. Examples of primary storage devices are RAM and cache memory. When a computer is turned off, the information stored in a primary device is lost. Secondary storage devices are used to store data permanently. They can be either internal or external to the computer, and they include the hard disk drive (HDD), solid state drive (SSD) or optical device such as DVD drive. Secondary storage retain their data even after the computer is powered off. When you are in an application and save your file, it transfers the file contents from RAM to a storage device such as HDD or SSD.
Optical drives are the slowest form of storage as data transfers to and from storage is much slower. Hard drives are much faster than optical storage. Both optical storage and HDD are fairly inexpensive. The fastest form of storage is SSD drives. It can be more than 10 times faster than HDD but is more expensive. From a capacity perspective, optical storage have small capacities and usually 4.7 GB for DVD, while Blu-ray have capacities of 25 GB for single layer and 50 GB for dual layer disc. 100 GB discs are now being manufactured by adding a third layer. SSD are becoming much bigger than before and range between 250 GB to 1 TB SSD. Meanwhile HDD have the highest capacity up to 10 TB. Please note that 1 TB is equal to 1000 GB.
Server platforms on the other hand require much greater capacities, and hence they use multiple HDD or SSD or a combination of SSD and HDD in an aggregate called RAID (Redundant Array of Independent Disks). This allows it to have much larger capacities.
Secondary storage may be internal to the computer such as HDD, DVD, SSD, or external to the computer such as removable USB flash drive, external SSD drives, external hard drive or external DVD drive. In addition, you may connect to a Network Attached Storage (NAS) whereby data is transferred over the network to a storage device. This NAS device could be on your local network or external network. A good example of external network storage is Cloud Storage such as Microsoft One-Drive or Apple iCloud Drive, Google Drive, etc.