Diagnosing FileVault Startup Disk Unlock Failures on macOS

Recognizable Condition

After logging in, the Mac shows a gray screen or returns repeatedly to the FileVault password prompt, preventing the desktop from loading.

Cause/Diagnostic Table

Ordered Checks

1. Verify the login password you are using matches the account password. Try logging in with another admin account if available, or reset the password via Apple ID.
2. If the password is correct but unlock fails, attempt to enter the FileVault recovery key at the preboot prompt.
3. Boot into macOS Recovery (press and hold Command+R at startup), open Disk Utility, select the startup APFS volume, and run First Aid.
4. Reset NVRAM/PRAM by shutting down, then powering on while holding Option+Command+P+R for about 20 seconds.
5. From macOS Recovery, choose Reinstall macOS. This reinstalls the system software while preserving user data.

Fixes Tied to Findings

• If the login password was incorrect, resetting it via Apple ID or another admin account restores normal unlock.
• If the recovery key is accepted, the disk decrypts and the login proceeds.
• If Disk Utility First Aid reports and repairs errors, the preboot volume corruption is resolved.
• If NVRAM reset clears stale boot arguments, the system can locate the preboot volume correctly.
• If a macOS reinstall completes without error, the startup disk unlocks normally on the next boot.

Escalation Criteria

If all five checks fail and the Mac still cannot unlock the FileVault volume, the data remains inaccessible. At this point:

• Verify you have a recent backup (Time Machine or other).
• Contact Apple Support or an Apple Authorized Service Provider for further diagnostics.
• Consider professional data‑recovery services if the backup is unavailable or incomplete.