Storage is the fourth step in the Information Processing Cycle. Main Memory is called primary storage since it is where the computer keeps what it is currently working on. Secondary storage is where the computer records results when it is finished. Data, programs, and results are stored in secondary storage because it is not volatile like main memory.
Magnetic hard disks are the most common type of secondary storage. The read/write head must never touch the spinning metal platters (a head crash) or you will get damage or even complete failure of the drive.
Magnetic disks are divided into circular tracks with sectors. Matching tracks on different platters form a cylinder. Several sectors are grouped into a cluster, which is the smallest division of the disk that can hold a single file. Any extra space in the cluster that did not get used (slack space) cannot be used by another file. Formatting a disk sets up these areas, erases any existing data, checks for physical and magnetic problem spots, and creates a root directory for keeping track of what file is using which clusters.
The capacity of a magnetic disk depends on how many tracks, how close together the bits are, and whether the drive can write to both sides of the disk.
Accessing data on a magnetic disk requires four steps. The read/write head must move to the proper track (seek), the disk must rotate to the proper sector (rotational delay), the read/write head must move down very near the disk (settle), the read/write head reads the data and sends it to the processor (data transfer). Times are measured in milliseconds (ms).
Solid state devices store data electronically instead of magnetically and have no moving parts. Solid state drives are super fast. They can fail without warning. The media will wear out eventually. Flash drives are the current best choice for removable media. Just don't lose yours!
Optical disks come in several types of CD and DVD. Commerical discs that are used to send out software are of the Write Once Read Many (WORM) type. Writable discs can be recorded on only once. Rewritable discs can have files written, erased, and rewritten many times. Data on such discs will last for a long time if care is taken to avoid high heat and sunlight and to keep them free of scratches. Data is written by changing a layer inside the disc. Data is read by shining a laser light onto the layer and seeing how that light bounces off.
Magnetic tape is still used for large data records, especially back ups. It is slow to record and very slow to recover a particular file.
You must take care of your data by using a good plan for making copies (back-ups) and for storing those copies away from where the computer is. Online storage, CDs or DVDs, flash drives, and external hard drives could all be part of your plan. You need multiple copies in mulitple locations for really important data. You can password-protect or encrypt files. You also need use antimalware software continuously.