Write-Blockers: Read Everything, Change Nothing

What you'll be able to do

By the end of this lesson you will be able to explain how write-blockers protect evidence, and use them with the right precautions.

• Describe hardware and software write-blockers and how each works.
• Explain the three functions of a write-blocker and why they matter in court.
• State what a write-blocker does not protect against, and how examiners test their equipment.

What a Write-Blocker Does

Chapter 7 describes write-blockers as essential tools that prevent data from being altered during an investigation. A write-blocker sits between the evidence and the examiner's computer and allows data to be read but blocks any attempt to write. That matters because ordinary computers write to drives constantly and automatically: updating timestamps, creating hidden system folders, indexing files. A write-blocker stops all of it reaching the evidence.

The book ties write-blockers directly to the chain of custody: the legal record of how evidence was handled from collection to court. If nothing could be written to the evidence, the record of its handling is far easier to defend.

Hardware and Software Write-Blockers

Hardware write-blockers are physical devices between the storage media (hard drives, SSDs, USB flash drives) and the analysis computer. The book describes two kinds:

• Tabletop units, standalone devices with several ports for different drive connections, such as SATA and the older IDE.
• Portable units, compact versions that are easy to carry to a scene and connect to a laptop.

Software write-blockers are programs that block write access on the computer used for analysis. The book mentions write-protection tools for operating systems such as Linux and Windows, and forensic suites with built-in write protection; it cites FTK Imager as an example.

A caution about software protection: a program that only reads a device does not by itself stop the operating system from writing to it when the device is connected. That is why many laboratories prefer a hardware write-blocker for physical media, and test any software method before relying on it.

Three Functions

The book identifies three functions:

1. Prevents data modification. No data can be written to the device, which also prevents accidental deletion or modification of files.
2. Maintains evidence integrity. The evidence stays in its original state for presentation in court, meeting legal standards of preservation.
3. Supports the chain of custody. Because the evidence cannot be altered, the record of who handled it and when is not undermined by doubts about changes.

Why They Matter

The book gives four reasons:

• Admissibility. Removing the risk of accidental change makes it more likely the evidence will be accepted in court.
• Malware. Write-blockers prevent anything, including malware from the examiner's workstation, from being written back to the evidence device.
• Preventing mistakes. Examiners under pressure make mistakes. A write-blocker means that even an accidental write attempt is blocked.
• Regulatory compliance. In some jurisdictions, standards require write-blockers, and failing to use one could get evidence challenged or dismissed.

The book lists where they are used: forensic investigations, data recovery from damaged or failing devices, incident response on suspect systems, and e-discovery, the legal process of collecting electronic evidence in civil cases.

Real-World Example

Imagine (a fictional scenario) an examiner preparing to image a suspect's external hard drive. Before connecting the evidence, she tests her hardware write-blocker. She connects a spare test drive through it, hashes the drive, tries to create a file on it (the attempt fails, as it should), hashes the drive again, and confirms the two values match. She records the test, the write-blocker's model and firmware version, and the result in her case notes. Then she connects the evidence drive, images it and verifies the hash.

In court, the defense asks how she knows her equipment did not alter the drive. She can point to the record of the test, made the same day, and to the matching hashes.

From the Book to 2026

What the book teaches. Hardware and software write-blockers prevent modification, maintain integrity and support the chain of custody. They are used in investigations, recovery, incident response and e-discovery.

Why it made sense. Write-blocking is still one of the foundations of forensic acquisition.

What has changed. Practice has become more exact about what a write-blocker does and does not do. NIST's Computer Forensics Tool Testing program publishes test methods for both hardware and software write-blocking, because blocking must be tested, not assumed. A write-blocker stops the host computer writing to the evidence. It does not stop an SSD's own controller from carrying out internal garbage collection while it has power (Lesson 2.3), and it does not make malicious files safe to open: examiners still analyze evidence on isolated workstations. And for phones and cloud accounts, there is no write-blocker to plug in; integrity relies on isolation, specialized extraction tools, hashing and documentation.

What to do now. Test your write-blocker before each use and record the test. Record the model and firmware version. Prefer hardware write-blocking for physical drives. Remember its limits with SSDs, malware and mobile devices.

Watch: Hardware Write Blockers

In this DFIRScience video, from a channel run by a digital forensics academic, the presenter explains hardware write blockers for forensic data acquisition.

Questions to keep in mind as you watch:

• How is the write-blocker connected between the evidence and the examiner's computer?
• What checks are done to confirm it is working before evidence is connected?
• What special care might flash-based devices such as USB sticks and SSDs need?

The video complements what you just read. The lesson is complete without it.

What Would You Do?

A colleague is in a hurry. The lab's hardware write-blocker is in use, so he plans to plug a suspect's USB stick directly into his workstation and image it with forensic software, "because the software is read-only anyway". What do you tell him? Decide, then compare with the model answer in the note below.

Knowledge Check

Key Takeaways

• A write-blocker lets the examiner read evidence while blocking every write: hardware (tabletop or portable) or software.
• Its three functions are preventing modification, maintaining integrity and supporting the chain of custody.
• Test it before use and record the test. NIST CFTT publishes test methods for hardware and software write-blocking.
• Know its limits: SSD internal housekeeping, malicious content and mobile devices need other safeguards.

Sources

• Dr. Tahseen Al-Doori — IT Forensics Essentials: Concepts, Tools, and the Practice of Digital Investigation, Chapter 7
• NIST — Computer Forensics Tool Testing Program (CFTT), including hardware and software write-block testing
• NIST — SP 800-86: Guide to Integrating Forensic Techniques into Incident Response
• NIST — IR 8354: Digital Investigation Techniques: A NIST Scientific Foundation Review (November 2022)
• arXiv — Hadi, Ullah and Harris, SSD Forensic: Evidence Generation and Forensic Research on Solid State Drives Using Trim Analysis (2023)