What happens when files overwritten?

File overwriting refers to replacing the contents of an existing file with new data. When a file is overwritten, the original data in the file is deleted and cannot be accessed anymore (Lenovo). Overwriting files matters for several reasons:

  • It allows reuse of disk space by deleting old files and writing new data over them (Minitool).
  • It helps prevent unauthorized access to sensitive data by ensuring old versions are securely deleted (Lenovo).
  • It enables updating files with new information by replacing outdated data (Bizmanualz).

In summary, file overwriting is the process of replacing old file contents with new data for purposes of reuse, security, and updating.

Original File Data

Hard disk drives store data in a logical format called sectors. Each sector holds a fixed amount of data, typically 512 bytes. The hard drive consists of platters which are coated in a magnetic recording material. Data is written to each platter in concentric tracks which are divided into the individual sectors. An actuator arm with a read/write head for each platter moves across the surface to access the desired location for reading or writing data. As new data is written, it overwrites any existing data stored in those sectors (Degassing-101.com, 2019).

The hard drive relies on low level firmware and the file system to keep track of where data is physically located on the drive. The firmware handles the mechanics of accessing the correct sectors while the file system manages the logical storage of files in available sectors. This allows files to be stored non-contiguously and prevents data from being overwritten unintentionally. However, when a file is deleted, those sectors are marked as available space and any future writes can overwrite that area (Stuffedcow Blog, 2019).

File Overwrite Process

At a low level, overwriting a file involves opening the file, seeking to the beginning, writing new data until the end of file is reached, and closing the file. This replaces the existing data in the file with the new data (1).

In most operating systems, overwriting a file simply replaces the file’s entry in the file table with the new data, without actually deleting or overwriting the data on disk. To ensure the original data is overwritten at the physical level, special utilities are required that access the disk directly and force rewriting of the specific sectors (2).

Common Misconceptions

There are some common myths and misconceptions when it comes to overwriting files. One myth is that when a file is deleted or overwritten on a hard drive, it is completely erased and unrecoverable. However, this is often not the case. When a file is deleted, typically only the reference to that file in the file system table is removed, not the actual contents of the file itself. The data still resides on the hard drive until it is overwritten by new data.

Another misconception is that formatting a hard drive or reinstalling the operating system deletes all files and makes them unrecoverable. Again, formatting a drive does not actually overwrite the existing data, it only removes the file system structure. The original data still remains intact until overwritten.

There are advanced techniques that can potentially recover overwritten files by analyzing magnetic residue on the physical drive platters. However, actually recovering usable data from overwritten files is quite challenging, expensive, and usually only partially successful. The likelihood of meaningful file recovery depends on the number of overwrites, the type of overwrite method used, and the time elapsed since the overwriting occurred.

(Overwriting Hard Drive Data: The Great Wiping Controversy, 2008;
Misconceptions around instant file initialization, 2009)

Recovering Overwritten Files

In some cases it may be possible to recover an overwritten file, but there are no guarantees. When a file is overwritten, the original data is not erased immediately. Some of the data may still exist until it is fully overwritten by new data. Several tools and techniques exist that can help recover portions of overwritten files by scanning the storage device to find traces of the old data.

One method is using file recovery software that searches for file signatures or patterns related to the old file. Sometimes intact portions of the overwritten file may still reside in slack space or unallocated clusters. File recovery utilities can piece these fragments together to recover at least partial versions of overwritten files.

Another potential technique involves forensic file recovery using magnetic force microscopy. This scans the physical storage device and attempts to reconstruct some of the old data based on magnetic traces left on the hardware itself. However, this approach requires specialized equipment and expertise.

The likelihood of recovering an overwritten file also depends on the number of passes used to overwrite the data. The more iterations, the less likely that any traces of the original data remain. Additionally, solid state drives store data differently than traditional hard drives, making recovery from SSDs very difficult.

While recovering overwritten files is sometimes possible, it is an inexact process with no guarantee of success. The best practice is to always maintain backups of important files to avoid needing to attempt such recovery in the first place.

Overwrite Methods

There are two main methods for overwriting files: single overwrite and multiple overwrite. With a single overwrite, the new data is written over the existing data just once. This means remnants of the original data may still exist on the drive since areas containing 1s may simply be switched to 0s without erasing what was there before. A single overwrite is fairly quick but is not considered a secure method for eliminating sensitive data.

Multiple overwrites are more secure. This technique writes new meaningless data on top of the existing data multiple times to ensure no trace of the original data remains. The US Department of Defense recommends overwriting data at least 3 times for proper sanitization, with 7 passes considered the most secure [1]. The more wipes, the less likely any original data could be recovered by advanced forensic analysis. However, it also takes much longer to perform multiple overwrite passes.

Some overwrite utilities offer different algorithms for the data written during each pass. Pseudorandom data is considered more secure than simple 1s and 0s. The US standards specify overwriting first with all 0s, then all 1s, then a random character.

Overwrite Utilities

There are various software tools available that are designed to securely overwrite files by overwriting the existing data. Some common overwrite utilities include:

Shred – A Linux command line tool that overwrites files multiple times with pseudo-random data making it very difficult to recover the original contents.

Eraser – An open source Windows application that overwrites data multiple times meeting government standards for secure data wiping.

WipeFile – A free utility for Windows that overwrites files making recovery virtually impossible. It utilizes military-grade wiping algorithms.

File Shredder – A Mac program that overwrites files and folders repeatedly to prevent recovery. It conforms to US Department of Defense erasure standards.

These types of dedicated file wiping utilities use multiple overwrite passes and standards like DoD 5220.22-M to ensure previously deleted files cannot be recovered by forensic analysis.

Secure Deletion

Secure deletion specifically aims to prevent recovery of deleted files and overwrite data beyond recovery, unlike basic overwriting. Standard overwriting methods can potentially leave remnants of file data that could be recovered forensically, but secure deletion is designed to prevent this.

Secure deletion utilities employ advanced overwrite techniques that completely replace data and make recovery impossible. Methods like Gutmann’s 35-pass overwrite and Peter Gutmann’s algorithm write randomized data patterns across all sectors to guarantee erased files can never be retrieved. According to the University of Michigan Securely Delete Files, secure deletion runs “special algorithms to repeatedly overwrite the area of the disk where a deleted file resided.”

The U.S. Cybersecurity and Infrastructure Security Agency also recommends secure erase tools that “overwrite all areas of the hard drive” to permanently erase data. Secure deletion overwrites every disk sector, while basic overwriting may just write over file clusters. This prevents any chance of forensic recovery.

In summary, secure deletion provides a complete overwrite to guarantee erased files cannot be recovered, while basic overwriting may leave recoverable remnants. Secure delete aims to fully prevent recovery through advanced multi-pass algorithms.

Overwriting vs Other Operations

Overwriting files is different from other common operations like deleting or formatting a drive.

When you delete a file, the reference to the file’s data on the disk is removed, but the actual data still remains on the drive. The space the file occupied is simply marked as available to be overwritten. Until new data occupies that space, recovery software can often restore deleted files.

Formatting or partitioning a drive does not actually erase any data either. It resets the file system, clearing the list of files on the drive. However the raw underlying data still remains intact until it gets overwritten. Formatting a drive is not a secure method of erasure.

Overwriting, on the other hand, directly replaces old data with new meaningless data. It actively wipes sensitive information by recording over it multiple times to obscure the original data. Once data has been completely overwritten, no traces of the old data can be recovered – not even with advanced forensic analysis.

For permanently erasing sensitive information, overwriting data is more effective than simply deleting files or reformatting drives.


In conclusion, overwriting files is a common data management process that deletes old data and replaces it with new data in the same storage space. However, critical files can be permanently erased if proper precautions are not taken. It’s important to understand that modern storage devices do not always fully overwrite data, leaving remnants behind. Recovering overwritten files is possible in some cases using advanced forensic tools that scan storage media. To prevent accidental data loss, system administrators should utilize secure deletion utilities and establish overwrite policies. Some key learnings are:

  • Overwriting does not always fully replace original file data due to disk geometries.
  • With the right tools, remnants of overwritten files can sometimes be recovered.
  • Multiple overwrite passes are required for secure deletion of sensitive files.
  • Dedicated overwrite utilities should be used to purge deleted files.
  • Organizations need formal procedures for overwriting files and storage devices.

By understanding the risks of overwriting and implementing proper protocols, companies can avoid critical data loss and unauthorized data recovery.

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