Data is retained in persistent storage by being written onto non-volatile storage devices, such as hard drives, solid-state drives (SSDs), or flash drives.

To elaborate, persistent storage refers to any device that retains data even after the power to the system is turned off. This contrasts with volatile memory, like RAM, which loses all stored information once the power is cut. Examples of persistent storage devices include hard disk drives (HDDs), SSDs, flash drives, and optical discs such as CDs and DVDs.

The methods for data retention vary according to the type of storage device. For instance, in a hard drive, data is stored magnetically on rotating platters. When data is written to the drive, the read/write head alters the magnetic orientation of specific regions on the platter to represent binary data (1s and 0s). This magnetic orientation remains intact even when the drive is powered off, allowing the data to be preserved.

In contrast, solid-state drives store data in flash memory cells that hold an electrical charge to represent binary data. Unlike the magnetic storage in hard drives, flash memory cells can retain their charge without a power supply, which is what makes SSDs non-volatile. Consequently, SSDs are typically faster and more reliable than HDDs, though they tend to be more expensive.

Flash drives operate similarly to SSDs, utilizing flash memory for data storage. However, they are generally smaller and more portable, making them ideal for transferring data between various devices.

Optical discs, such as CDs and DVDs, store data by employing a laser to create pits in a reflective surface. These pits, along with the spaces between them, represent binary data. Once the data is etched onto the disc, it can be read by a laser, even when the disc is not powered.

In all these scenarios, the fundamental principle of data retention in persistent storage is the use of non-volatile methods that can maintain data without a continuous power supply. This ability to store data long-term and access it as needed makes persistent storage an essential component of any computer system.