The alternate partition is corrupt and the following error is observed:

The following error is seen on the alternate partition:

The alternate root partition might have been corrupted, which lead to this error condition.

1. Check the traces of bad blocks:

   INCORRECT BLOCK COUNT I=133 (156 should be 152)
   CORRECT? no
   
   ** Phase 2 - Check Pathnames
   ** Phase 3 - Check Connectivity
   ** Phase 4 - Check Reference Counts
   ** Phase 5 - Check Cyl groups
   FREE BLK COUNT(S) WRONG IN SUPERBLK
   SALVAGE? no
   
   SUMMARY INFORMATION BAD
   SALVAGE? no
   
   BLK(S) MISSING IN BIT MAPS
   SALVAGE? no

2. Perform nand-mediack checks and clear to clean up the file system:

   root@uer02:RE:0% nand-mediack -C
   Media check on da0 on ex platforms
   root@uer02:RE:0%
   root@uer02:RE:0% nand-mediack
   Media check on da0 on ex platforms

3. Proceed with one system reboot.

4. After the reboot, if the error still exists, un-mount the temporary mount point for the alternate partition:

   root@uer02:RE:0% fsck -f -y /dev/da0s2a
   ** /dev/da0s2a
   ** Last Mounted on /tmp/snap-tmp.1736/mnt.1736 <<<<<<
   ** Phase 1 - Check Blocks and Sizes
   ** Phase 2 - Check Pathnames
   ** Phase 3 - Check Connectivity
   ** Phase 4 - Check Reference Counts
   ** Phase 5 - Check Cyl groups
   4 files, 4 used, 94098 free (34 frags, 11758 blocks, 0.0% fragmentation)
   
   root@uer02:RE:0% umount /tmp/snap-tmp.1736/mnt.1736
   umount: /tmp/snap-tmp.1736/mnt.1736: statfs: No such file or directory
   umount: /tmp/snap-tmp.1736/mnt.1736: unknown file system

5. The error still exists:

   error: /dev/da0s2a is not a JUNOS snapshot

6. Snapshot slice alternate formatted the alternate root partition slice da0s2a 

   root> request system snapshot media internal slice alternate
   fpc0:
   --------------------------------------------------------------------------
   Formatting alternate root (/dev/da0s2a)...
   Copying '/dev/da0s1a' to '/dev/da0s2a' .. (this may take a few minutes)
   The following filesystems were archived: /
   
   {master:0}
   root> show system snapshot media internal
   fpc0:
   --------------------------------------------------------------------------
   Information for snapshot on       internal (/dev/da0s1a) (primary)
   Creation date: Nov 12 13:48:58 2018
   JUNOS version on snapshot:
   jbase  : ex-12.3R9.4
   jkernel-ex: 12.3R9.4
   jcrypto-ex: 12.3R9.4
   jdocs-ex: 12.3R9.4
   jswitch-ex: 12.3R9.4
   jpfe-ex42x: 12.3R9.4
   jroute-ex: 12.3R9.4
   jweb-ex: 12.3R9.4
   fips-mode-powerpc: 12.3R9.4
   Information for snapshot on       internal (/dev/da0s2a) (backup)
   Creation date: Dec 12 17:51:58 2018
   JUNOS version on snapshot:
   jbase  : ex-12.3R9.4
   jkernel-ex: 12.3R9.4
   jcrypto-ex: 12.3R9.4
   jdocs-ex: 12.3R9.4
   jswitch-ex: 12.3R9.4
   jpfe-ex42x: 12.3R9.4
   jroute-ex: 12.3R9.4
   jweb-ex: 12.3R9.4
   fips-mode-powerpc: 12.3R9.4