CHKDSK, fsck and First Aid: Repair vs. Recovery
CHKDSK with repair options, fsck when allowed to modify the file system, and Disk Utility First Aid are consistency/repair operations rather than data-recovery procedures. Their objective is a structurally consistent file system, not preservation of every previous metadata state. On healthy storage with a current backup, repair can be exactly the right operation; on the only copy of important data, recovery has a different priority.
Consistency can require discarding information
Depending on the file system and repair mode, a utility may reconnect or free orphaned objects, rebuild indexes, change allocation metadata, replay journaled changes, or discard structures it considers invalid. Those changes can improve consistency while making the pre-repair state harder to reconstruct.
Hardware condition comes first
Repair is especially inappropriate as a first step on a drive with read errors, abnormal delays, resets or mechanical symptoms. The utility may perform many reads and writes while the source is deteriorating.
A safer order for important data
Preserve the source or create a sector-level image first. Perform recovery against a copy when practical. Once important files are independently verified, repair becomes a maintenance decision rather than a recovery gamble.
When repair is reasonable
If storage is healthy, data is backed up, and the goal is to restore normal consistency rather than recover a previous state, the platform's repair utility is the normal tool. The warning is about using repair as the first recovery experiment.
Repair asks: “Can this file system become consistent again?” Recovery asks: “What information can still be extracted from the current state?” Those goals overlap, but they are not identical.
Platform references: Microsoft CHKDSK documentation · Apple Disk Utility First Aid
Related: File System Corruption · APFS Corruption · Disk Imaging vs. Cloning