Start with the original bytes
Keep an untouched copy of the device file before trying any recovery. The header gives the declared data size and the message stream contains local definitions followed by records. A truncated transfer can leave the header intact while removing the final messages or checksum. A damaged byte can also break a definition or alter a sensor value without making every later field unreadable.
This desk reports header and footer CRC status separately. An unavailable footer is different from a mismatched footer. Neither result is silently treated as a fully healthy activity. The audit also records the number of retained, discarded, trailing and missing declared bytes so a recovery decision can be tied to the actual source.
Recover a complete prefix
The parser follows the message definitions in order, including byte order, native fields and developer-field lengths. It stops at the first incomplete or malformed message. It does not scan later bytes for patterns that merely resemble a record header, because that can produce convincing but false measurements. Complete earlier messages remain available.
Rebuilding writes a new header and CRC around that unchanged prefix. Unknown and developer message bytes are preserved even when the workbench cannot interpret them as sensor columns. This preserves more evidence than reconstructing a FIT file from a narrow CSV projection. It still cannot recover information that was never present in the available bytes.
Check activity requirements separately
A complete FIT activity needs file identification, records and lap, session and activity summaries. A device may write important summary messages only when recording finishes. Losing the tail can therefore leave a useful track but an incomplete activity container. The workbench flags missing message types and inconsistent summary timing rather than inventing plausible totals.
Try a recovered copy in the intended application only after reviewing the audit. Keep the original file and the recovery report together. If the platform refuses an incomplete FIT, the selected GPX or CSV can still be useful for viewing the surviving track or measurements. Acceptance by one application does not certify the accuracy or completeness of the recovered activity.