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First, you need to understand what data is. Data stored on computers and phones is digital information comprised of a bunch of ones and zeroes (called bits) that are recorded using a variety of methods ranging from magnetic charges (hard drives, floppy drives, magnetic tape), pits etched into a surface/substrate (CDs/DVDs/Blu-Rays), or even electric charges inside of a transistor (SSDs).
You also need to understand how these digital bits are arranged to form a piece of data like a file or a database and what technique is used to “delete” them when such a command is issued.
In the case of files on a storage device like a hard drive, flash drive, or SSD, a file is a bunch of bits (known as storage blocks) that are stored on the device and a special set of files called a directory is updated to remember which blocks belong to which file and where they are on the storage device. When a file is deleted through an operating system, the directory entry for the file is simply removed and the blocks of bits that comprise the file are left in place. But since there’s no longer a record of them being used, they’re free to be overwritten by the next file that needs to be saved.
Databases aren’t much different. There are other ways they’re organized called pages and records. And there’s an index that tracks where each record is. When a record is deleted, many database systems leave the record in place and delete the index entry much like files on a drive are deleted. Often, database owners will run a cleanup command to re-write the data in a database to remove these unused records to optimize performance.
Other mediums like tapes and re-writable optical work similarly even though their structure is quite different.
As you might surmise, there are points in time after which data has been “deleted” that it might still be recoverable because only the directory or index of the data has been removed. This is true and specialized data recovery software can do just that.
Even on drives that use encryption, deleted files can be recovered if the drive has been unlocked using its encryption key.
Some operating systems include a secure erase option that overwrites the deleted files with random data to make recovery impossible.
When discarding a storage device that you want to make sure can no longer be recovered, it is important to use a wiping tool to overwrite the entire drive with random bits and/or zero it out with empty bits. Alternatively, if the drive was encrypted, erasing only the key stored on the drive will make it effectively unreadable with today’s technology.
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