We may observe the Next-Generation Routing Engine (NG-RE) boots from the alternate disk (Disk2) on the chassis alarms. This article explains how to troubleshoot this problem.

When a primary disk fails, the Routing Engine (RE) boots from the alternate disk (Disk2) and we would be seeing the below alarm:

> show system alarms

1 alarms currently active

Alarm time Class Description

2024-08-07 18:36:03 UTC Minor VMHost 0 Boot from alternate disk

To checked if the Junos is booted from the primary/secondary disk, you have to execute the command "show vmhost version"

> show vmhost version
Current root details,          Device sdb, Label: jrootp_S, Partition: sdb3
Current boot disk: Secondary
Current root set: p
UEFI   Version: REH_P_MTR1_00.30.07
Secondary Disk, Snapshot Time: Fri Jul 14 10:29:09 IST 2023
Version: set p
VMHost Version: 5.2279
VMHost Root: vmhost-x86_64-20.2R3-S1-20210430_0731_builder
VMHost Core: vmhost-core-x86-64-20.2R3-S1.3
kernel: 4.8.28-rt10-WR9.0.0.24_ovp
Junos Disk: junos-install-mx-x86-64-20.2R3-S1.3
Version: set b
VMHost Version: 5.2279
VMHost Root: vmhost-x86_64-20.2R3-S1-20210430_0731_builder
VMHost Core: vmhost-core-x86-64-20.2R3-S1.3
kernel: 4.8.28-rt10-WR9.0.0.24_ovp
Junos Disk: junos-install-mx-x86-64-20.2R3-S1.3

To resolve the issue, we may need to follow the below workaround during the MW time:

• Run the command ">request vmhost snapshot recovery partition" and reboot the RE. 

> request vmhost snapshot recovery partition

warning: All existing data on the target media will be lost

Proceed ? [yes,no] (no) yes

Creating snapshot on target disk from current boot disk .

Primary disk is recovered now. Please issue "request vmhost reboot" to boot from the primary disk.

Software snapshot done

• Try USB recover, because if the issue is related any file system error/corruption. Then USB recover will format the disk and can fixed it the issue not related to hardware. 
• If the alarms are not cleared, then we may to try checking the RE with a spare (if available). If the newer RE doesn’t experience this alarm, then we can confirm the old RE is fault and the final step is to replace the RE to resolve the issue.