Evidence Preservation and Chain of Custody in Computer Forensics (Plain-English Guide)

Preservation + Chain of Custody Documentation • Photos • Handling • Transfers • Defensibility

Evidence Preservation and Chain of Custody (Why It Matters in Computer Forensics)

In computer forensics, the technical analysis is only as defensible as the evidence handling that came before it. Preservation and chain of custody are the foundation that allow computer forensic experts to explain what was done, who handled the evidence, and why the findings are reliable. For the complete service overview and related guides, see: computer forensics.

What this covers

This page explains how digital evidence should be documented, how to preserve devices and media to reduce accidental changes, and what a chain of custody form is (including the role it serves in maintaining integrity and credibility). This is informational guidance designed to help you understand what professional computer forensic companies typically require for a defensible workflow.

Evidence documentation: what to record immediately (device identifiers, condition, environment, and handling).
Photographs: what to capture and why photos matter for later interpretation.
Chain of custody: what the form is, what it tracks, and why it is important.
Handling decisions: powered-on vs powered-off considerations and why “do nothing” is often safest.
Common mistakes: actions that unintentionally alter timestamps, logs, and recoverable deleted space.

Internal navigation

If you need the “what is computer forensics” primer first, start here: what is computer forensics. If you want the next stage after preservation (imaging and acquisition), see: forensic imaging and acquisition.

Important: This page is educational. It is not legal advice, and it does not instruct anyone to access systems they are not authorized to access. A defensible approach documents what was observed and done, and avoids unnecessary changes.

The core preservation principle (minimize change, maximize documentation)

Digital evidence is unusually easy to change without noticing. Simply turning a device on, logging in, connecting it to Wi-Fi, or running “cleanup” utilities can create new artifacts, modify timestamps, rotate logs, and overwrite data that might otherwise be recoverable. That is why preservation practices prioritize:

Minimizing interaction with the original evidence source.
Documenting condition and context before any handling step.
Working from verified copies whenever the scope and access conditions allow (e.g., forensic imaging).

Practical takeaway: The fastest way to reduce defensibility is to “poke around” on the device to look for clues before the evidence is documented and preserved.

How evidence should be documented (what to record early)

Documentation supports repeatability and helps explain later why a particular artifact exists (or does not exist). This applies whether the evidence source is a desktop, laptop, server, external hard drive, USB flash drive, or SD card.

Device and media identifiers

Device type (desktop/laptop/server) and manufacturer/model (if known)
Serial numbers / asset tags (if present)
Drive identifiers (labeling on the device, enclosure, or packaging)
Storage type notes (HDD vs SSD vs flash media) when visible
Any obvious damage, missing screws, or tamper indicators

Condition and environment

Was the device powered on or off when received?
Was it logged in? If yes, what was visible on screen?
Were external devices connected (USB drives, SD cards, dongles)?
Network status (connected to Wi-Fi/Ethernet, airplane mode where applicable)
Date/time received and who received it

Why this matters

Many forensic conclusions depend on context. For example, whether a system was running or powered off can change what data is accessible, how encryption behaves, and whether certain logs continue to rotate. Good documentation protects the integrity of later analysis and reporting.

Photographs (what to capture and why it helps)

Photos are often the simplest way to preserve “state evidence” that may not be reconstructable later. They help show what was present at the time the device was received and can resolve disputes about handling steps. Photos should be clear and time-associated.

Overall device photos: front/back/sides, ports, labels, and any damage.
Screen state (if on): lock screen, desktop, open applications, error messages, and visible clocks/time zones.
Connected items: photos showing USB devices, dongles, cables, and external storage connected at intake.
Packaging condition: if shipped, photograph packaging before opening and internal packing materials.
Drive labels: any stickers or handwritten labels on drives or enclosures.

A common mistake is taking photos after interacting with the device. Whenever possible, photograph first, then proceed with minimal handling.

What is a chain of custody form? (and what role it serves)

A chain of custody form is a structured record that tracks the physical (and sometimes digital) control of evidence over time. It documents who had the evidence, when they had it, why it changed hands, and how it was stored or transported. In computer forensics, this record helps show that evidence was handled consistently and reduces arguments that the evidence was altered or substituted.

What it typically includes

Unique evidence ID and item description
Device/media identifiers (serial numbers, labels)
Date/time received and received-from information
Every transfer: released-by / received-by, date/time, purpose
Storage/handling notes (sealed, location, condition)

What it helps establish

Continuity: the same item from intake through analysis
Accountability: who had custody at each stage
Transparency: handling steps are visible and reviewable
Reduced dispute risk about tampering or substitution
Professional defensibility for computer forensic reporting

Chain of custody does not “prove” evidence content is true. It supports credibility by showing evidence handling was controlled and documented.

Practical handling guidance (common scenarios)

The safest handling steps depend on device state, encryption, and scope. The guiding idea is to reduce change and preserve options for imaging and analysis.

If the device is powered off

Power-off state can be protective: it prevents ongoing log rotation and reduces overwrite risk from normal system activity. However, encryption access may require credentials later.

Document the state and do not “test boot” casually
Package and store securely to preserve condition
Plan imaging steps based on scope and access

If the device is powered on

Powered-on systems can contain volatile context (open apps, active connections), but they can also change rapidly and may be encrypted in ways that shift on reboot.

Photograph the screen and visible time zone/clock
Avoid clicking through menus “to look around”
Consider whether scope requires live context vs minimal change

External media (USB/SD)

External hard drives, flash drives, and SD cards are often central to disputes. Their file systems and storage type affect metadata and deleted data realism.

Label items clearly and document each one separately
Do not plug into multiple systems “to see what’s on it”
Preserve packaging/labels and photograph both sides

Common mistakes that weaken defensibility

Many issues seen by computer forensic experts are not technical—they are handling problems that introduce doubt. These are common examples:

Logging in “just to check something” before intake photos and documentation are completed.
Running antivirus/cleanup utilities that modify or delete artifacts (sometimes silently).
Copying files off the device using the suspect OS environment, creating new timestamps and records.
Allowing the device to connect to the internet (updates, sync, log rotation, remote changes).
Untracked custody (no record of who had the device and when).
Mixing items (USB drives and adapters not labeled; multiple drives stored together without identifiers).

Good computer forensic companies document these risks and build a workflow that reduces them before imaging and analysis begin.

What typically happens next (after preservation)

Once evidence is documented and custody is tracked, the next step is usually acquisition (often forensic imaging) followed by parsing and analysis of artifacts. That sequence helps ensure findings are based on verifiable copies and the original evidence is handled minimally. For the broader overview of computer forensic services and how engagements are typically structured, see: computer forensics.

Imaging and acquisition explained What is computer forensics? Main computer forensics hub

Educational note: Preservation is about controlled handling and accurate documentation. It reduces avoidable disputes and supports more reliable reporting later.