Get-FileHash vs fc.exe for multi-TB restore verification: throughput vs early-exit trade-off

Direct Answer

For a 12 TB restore with 400,000 files, Get-FileHash -Algorithm SHA256 in PowerShell 7.4 is the superior choice because it generates a reusable manifest and supports Alternate Data Streams (ADS). While fc.exe offers an early-exit on mismatch, this only provides a speed advantage if the restore is already corrupted; for a successful restore, both tools must read the entire 12 TB. fc.exe also requires the source volume to be mounted and unchanged, creating a dependency that manifests avoid.

Likely Explanation vs. Confirmed Behavior

Likely Performance Dynamics:

• I/O Bottleneck: At 12 TB, the process will be almost entirely I/O-bound. Parallelism in PowerShell 7.4 helps saturate the storage pipeline, especially when processing the large volume of small files (1 MB range), but will not bypass the physical read limits of the disk array.
• Memory Pressure: Get-FileHash streams data, meaning 8 GB files will not cause Out-of-Memory (OOM) errors. fc.exe also streams binary comparisons rather than loading entire files into RAM, though it lacks the progress reporting and auditability of a hash list.

Confirmed Technical Facts:

• ForEach-Object -Parallel is exclusive to PowerShell 7+; PowerShell 5.1 is single-threaded for this operation.
• Get-FileHash -Stream allows direct hashing of Zone.Identifier without a separate file system pass.
• Compare-Object is safe for comparing CSV manifests (strings) even if the original files contained binary nulls.

Implementation Strategy

1. Verify Environment: Confirm $PSVersionTable.PSVersion is 7.4+.
2. Throughput Baseline: Measure sequential read speed on the target volume. Divide 12 TB by this rate to verify if the 4-hour window is physically possible.
3. Parallel Chunking: Use fixed file-count batches (e.g., 200 files per batch) piped to ForEach-Object -Parallel -ThrottleLimit 8. This prevents runspace pool exhaustion that can occur with unpredictable per-directory grouping.
4. ADS Integration: Within the parallel loop, use Get-Item -Stream * to identify the Zone.Identifier stream and hash it using Get-FileHash -Stream.
5. Manifest Comparison: Export results to CSVs for source and target. Use Compare-Object on the Hash and Path properties of the imported CSVs.

Critical Diagnostic Detail

Is the original source volume still accessible and frozen (read-only)? If the source is unavailable or has changed since the backup, fc.exe is technically impossible to use, making the manifest approach the only viable option.