Use recovery tools with a purpose

Each recovery tool answers one question, and using a tool outside its boundary produces answers that mean nothing.

Apple Diagnostics checks supported hardware: memory, power, sensors. It ends with a reference code — ADP000 means no issue found, anything else is a code to record verbatim and look up. It says nothing about software.

Disk Utility First Aid checks filesystem structures. It verifies and repairs APFS metadata. It cannot fix a misbehaving app, and a clean First Aid pass does not rule out a failing drive — it only rules out structural filesystem damage.

macOS Recovery provides repair, restore, and reinstallation paths outside the normal startup system. From there you can run Disk Utility on the startup disk, restore from Time Machine, or reinstall macOS.

Pick the tool the evidence points at

Match symptom to boundary. Kernel panics or shutdowns under load: run Apple Diagnostics first. Files failing with I/O errors, a volume that will not mount: First Aid, from Recovery so the startup disk is not in use. System files damaged but user data intact: a reinstall over the existing system.

Before any of them: back up recoverable data before repair when possible. Repairs can fail halfway, and a disk healthy enough to copy files from today may not be tomorrow. Record error codes and exact disk targets — “First Aid failed” is useless, “First Aid on disk3s5 exited with error 8 at 14:32” is evidence:

2026-08-03 14:32  First Aid on disk3s5 (Macintosh HD - Data)
result: exit code 8, "The volume could not be verified completely"
next: back up now, then retry from Recovery

Reinstalls and the SIP temptation

Do not erase or reinstall because a generic checklist places it first. A reinstall can replace system files while preserving the user problem that caused the symptom, so define the expected outcome before starting. If the fault lives in ~/Library, a reinstall changes nothing except your afternoon.

Recovery’s Terminal also offers csrutil, which can disable System Integrity Protection. Never do that as a troubleshooting step. No ordinary diagnosis requires it, and any guide that starts there is a guide to distrust.