RAID 5 Reconstruction with a Missing Disk
A 4-disk RAID 5 array is seized from a graphic design studio suspected of storing unlicensed software. One disk is physically failed and will not spin. The remaining three 2 TB SATA disks are imaged with SHA-256 hashes. The array controller is a Dell PERC H730, and the server was configured with a 64 KB stripe size, but the documentation is missing. The file system is NTFS. The examiner needs to reconstruct the array to examine the volume. The three images are 2 TB each; the failed disk's data must be inferred from parity. The examiner has EnCase, FTK Imager, and a Linux workstation with mdadm and sleuthkit. The case agent wants a report within 48 hours. The seized server was powered off at the scene, so no live memory is available.
1. What parameters must be determined before reconstruction, and how can they be inferred?
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Reasoning: A RAID 5 reconstruction requires the disk order, parity layout (left or right, symmetric or asymmetric), stripe or chunk size, start offset, and file-system offset. Getting any of these wrong produces a volume that may mount but contain corrupted files, so validation is essential.
The undocumented 64 KB stripe size is a hypothesis, not a fact. The examiner can infer parameters by examining controller metadata if present, looking for NTFS boot-sector signatures at candidate offsets, testing plausible disk orders and parity rotations, and checking $MFT record signatures after assembly. mdadm --examine and file-system tools such as fsstat can help validate candidate assemblies. Known files, directory structure, and file signatures should be used as checks.
The examiner should record every attempted combination and its result. The goal is not simply to mount something, but to produce a defensible reconstruction that can be explained and reproduced.
2. Outline a defensible reconstruction and validation procedure.
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Reasoning: Work only on verified copies of the three disk images. Confirm the SHA-256 hashes, record tool versions, and create a degraded RAID 5 array with one missing device using mdadm. Set the candidate chunk size and parity layout, then assemble the array read-only. Mount the NTFS volume read-only with ntfs-3g or examine it with sleuthkit.
Validation should include checking the boot sector, $MFT, directory tree, and known file signatures. Extract sample files and verify that their headers and content are coherent. If the volume does not validate, try other plausible disk orders or parity layouts, but do not present a failed assembly as evidence. Hash the reconstructed image and document the exact commands.
The report should explain that one disk was missing and that its data was inferred from parity. It should disclose assumptions, failed attempts, and any areas that could not be verified. A reconstruction that mounts but has corrupted file content is worse than a documented failure.
3. How should the report address the missing disk and the use of parity?
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Reasoning: The report must be explicit that only three of four disks were imaged and that the failed disk was reconstructed from RAID 5 parity. It should not imply that the examiner examined the failed disk directly. The report should describe the controller model, the inferred layout, the stripe size, the tools used, and the validation checks.
The report should also state the limitations: parity reconstruction can recover data only if the remaining disks are consistent and the failed disk's data is fully reconstructable. Silent corruption, controller metadata, or an incorrect parameter can produce plausible but wrong output. Any files that failed validation should be identified.
The examiner should use language such as 'consistent with' and 'reconstructed from parity' rather than 'the missing disk contained.' The report should include hashes, chain of custody, and a clear statement that the reconstruction is an expert interpretation, not a perfect copy of the original array.