Error observed:
In the NG-RE, there are two SSD disks, ssd1 and ssd2. SSD1 is the primary disk and SSD2 is the backup. There are two SETs on each disk. We boot with one set and when performing upgrade, the new Junos image is installed on the other SET and vice versa.
However, getting the above mentioned error log is due to having different Linux versions WRL6 and WRL9 on the two sets of disks. Since the snapshot is successful, the error is considered harmless.
Primary Disk, Upgrade Time: Tue Apr 6 11:52:22 UTC 2021