Hash Mismatch on a Seized USB Drive
At 09:15 on 14 March, a field officer seized a 64 GB USB flash drive from a locked desk drawer in a shared office. The officer connected it to a validated write-blocker, imaged it to a 128 GB forensic SSD, and recorded SHA-256 3f9a...c21e. The drive was sealed in an anti-static bag, logged, and transported to the lab. On 17 March, a lab examiner opened the bag, noted the seal intact, and recomputed the hash of the stored image as 8b04...7d6f. The lab also imaged the original drive again and obtained a third hash, 3f9a...c21e. The field notes say the write-blocker displayed no errors, but the officer admits the USB drive was 'warm' and had been used to copy a presentation earlier that morning. The case concerns exfiltration of client records. The prosecutor asks whether the evidence is usable.
1. What are the most likely explanations for the mismatch, and what should the examiner do before drawing a conclusion?
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Reasoning: There are three hashes to separate: H1 = field hash of the original drive, H2 = lab hash of the stored image, H3 = lab re-image hash of the original drive. Because H3 equals H1, the original drive content is consistent with what the field officer hashed. The discrepancy is between H1 and H2, so the first focus should be the stored image or its transfer, not necessarily the drive itself.
Likely causes include a transcription error in the field notes, hashing the wrong file in the lab (for example, a container rather than the raw image), bit rot or a write error on the forensic SSD, an imaging-tool error, or a write-blocker problem during the original acquisition. The fact that the re-image matches H1 makes a persistent write-blocker failure less likely.
Before concluding anything, the examiner should re-hash the stored image with at least two tools, verify file size and tool logs, inspect the forensic SSD health, and re-image the original drive to a fresh verified destination while hashing both source and destination. All original media and logs should be preserved. The examiner should document the discrepancy immediately and not overwrite the suspect image.
2. Does the mismatch automatically make the evidence inadmissible? Explain the factors a court or reviewer would weigh.
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Reasoning: A hash mismatch is a serious integrity issue, but it is not an automatic bar to admissibility. Courts generally weigh authenticity, reliability, and chain of custody rather than applying a single technical rule. If the original drive is available and H3 matches H1, the prosecution can authenticate the original and a new image. The H2 mismatch can then be explained as a storage or transcription error rather than evidence tampering.
The reviewer would consider whether the discrepancy was documented promptly, whether the original was preserved, whether the imaging and hashing tools were validated, whether the examiner can reproduce the field hash, and whether any gap in custody is explained. A chain-of-custody defect usually affects weight, not necessarily admissibility. However, if the original drive is unavailable and H1 cannot be reproduced, the stored image is much harder to authenticate.
The examiner should prepare a clear technical explanation, preserve all logs, and disclose the issue to the prosecutor. The safest position is not 'the evidence is unusable' but 'the stored image is suspect; the original re-image is consistent with the field hash, and the discrepancy must be explained on the record.'
3. Draft the key documentation entries you would make.
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Documentation entries:
- Date, time, location, and identity of every person handling the drive.
- Seal number and condition on receipt; photo of the sealed bag before opening.
- H1, H2, and H3 values with the exact file or device each hash covers.
- Make, model, and version of the write-blocker, imaging tool, and hashing utility.
- Size and path of the stored image; SMART or health status of the forensic SSD.
- Exact description of the mismatch and the time it was discovered.
- Re-hash results from at least two tools; re-image procedure and destination media.
- Notification to the case agent and prosecutor; any decision not to use the stored image.
- Statement that the original drive was not altered and that all work was done on copies or with a write-blocker.
- Any corrective action, such as re-imaging to new media or retiring the suspect SSD.
Reasoning: These entries preserve the ability to reconstruct what happened and distinguish a clerical or storage error from evidence tampering. They also give the reviewer enough detail to evaluate the chain of custody without relying on memory.