What this article covers

Explains how to identify onboard flash storage file system corruption, kernel read-only remount errors and boot block read errors on FortiGate appliances.

ScopeAll FortiGate models experiencing configuration save failures, system log errors (`EXT4-fs error`), or boot loops during startup.

Before using the CLI

Console accessPrepare a serial console before bootloader or TFTP recovery.
Configuration backupPreserve the latest valid configuration before destructive work.
Maintenance windowPlan for reboot, outage and possible re-imaging.

On-site workflow

01

Detect early flash failure: Zombie miglogd & 0WA I/O wait spike

Before flash storage permanently locks into read-only mode, failing eMMC/SSD flash exhibits distinct signatures: in diagnose sys top, miglogd transitions to State Z (Zombie) or D (uninterruptible sleep), CPU I/O wait (0WA) spikes dramatically (30% to 70%+), and Cached RAM inflates as write buffers stall.

diagnose sys top 2 50
# Check miglogd state (Z/D) and CPU 0WA percentage
diagnose hardware sysinfo memory | grep -i cached
diagnose sys disk status
02

Inspect crash, kernel remount & EXT4 block errors

Look for EXT4-fs errors (e.g. ext4_lookup), I/O errors (e.g. Buffer I/O error on device), and kernel messages remounting partitions read-only.

diagnose debug crashlog read
# Check for: EXT4-fs error or Remounting filesystem read-only
03

Validate the recovery command before use

The supplied source lists a filesystem scan or repair procedure using the following commands. Confirm the exact model, target disk and command effect with current documentation or TAC before execution.

execute formatlogdisk
execute disk scan
Stop and validate before continuing

These operations may erase log storage, modify filesystems or require a reboot. Do not run them as routine diagnostics.

04

Use console and bootloader recovery when required

If the boot filesystem is corrupted, the supplied source directs the engineer to interrupt the BIOS boot sequence and re-flash FortiOS with TFTP from the bootloader menu.

Stop and validate before continuing

Bootloader recovery is outage-causing and can remove the installed image or configuration. Use a verified image, console access and a rollback plan.

05

Escalate persistent read-only flash

When flash remains read-only after the supported recovery process, initiate an RMA request for hardware replacement.

Operational caution

Review before applying: Formatting, disk repair and bootloader re-flashing are high-risk recovery actions. They require a maintenance window, console access, verified backups and model-specific validation.
Next step

Preserve the hardware evidence

When the appliance continues to remount storage read-only, collect the hardware RMA evidence package rather than repeatedly formatting or re-imaging it.

Browse On-Site Tips Explore Ferrite workflows