Hi,
I had a similar problem with some standalone EX2200. About two weeks after I mirrored the snapshot to the alternate partition the switches were not able to mount those partitions again.
What fixed it for me was to simply "request system snapshot slice alternate" again to recover from this mount issue.
How have you done the snapshot? It seemed to me that this issue occured when I simply terminated my ssh session to the switch while the snapshot was still in progress (To wait 10 minutes for each switches snapshot is not an option if you have about 50...). I do not know how this mechanism works internally but maybe it needs the executing session to remain open. However the second time I kept all sessions open and until now none of the switches failed to mount the backup partition again.
Some outputs:
show system snapshot media internal
Information for snapshot on internal (/dev/da0s1a) (primary)
Creation date: Oct 13 07:07:27 2012
JUNOS version on snapshot:
jbase : ex-11.4R5.5
jcrypto-ex: 11.4R5.5
jdocs-ex: 11.4R5.5
jroute-ex: 11.4R5.5
jswitch-ex: 11.4R5.5
jweb-ex: 11.4R5.5
error: cannot mount /dev/da0s2a
show log snapshot
/sbin/snapshot-dual-root -X --target-slice alternate
Snapshot source: internal (/dev/da0)
Snapshot target: external (/dev/da1)
DUMP: WARNING: should use -L when dumping live read-write filesystems!
cannot open /dev/tty: Device not configured
DUMP: Date of this level 0 dump: Wed Nov 7 09:10:09 2012
DUMP: WARNING: no file `/etc/dumpdates', making an empty one
DUMP: Date of last level 0 dump: the epoch
DUMP: Dumping /dev/da0s1a (/) to standard output
DUMP: mapping (Pass I) [regular files]
DUMP: mapping (Pass II) [directories]
DUMP: estimated 97730 tape blocks.
DUMP: dumping (Pass III) [directories]
DUMP: dumping (Pass IV) [regular files]
warning: ./.snap: File exists
DUMP: 97.86% done, finished in 0:00 at Wed Nov 7 09:15:16 2012
DUMP: DUMP: 99074 tape blocks
DUMP: finished in 405 seconds, throughput 244 KBytes/sec
DUMP: level 0 dump on Wed Nov 7 09:10:09 2012
DUMP: DUMP IS DONE // I think this was the end of the file after the first snapshot was taken (the one where I terminated the session)
/sbin/snapshot-dual-root -X --info internal
Snapshot source: internal (/dev/da0)
Snapshot target: internal (/dev/da0)
1+0 records in
1+0 records out
512 bytes transferred in 0.002374 secs (215676 bytes/sec)
** /dev/da0s1a (NO WRITE)
** Last Mounted on /
** Root file system
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
1449 files, 48305 used, 45285 free (29 frags, 5657 blocks, 0.0% fragmentation)
umount: /tmp/snap-tmp.61334/mnt.61334: not a file system root directory
** /dev/altroot (NO WRITE)
** Last Mounted on /altroot
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
1446 files, 48348 used, 45242 free (26 frags, 5652 blocks, 0.0% fragmentation)
mount: /dev/da0s2a : Operation not permitted // This propably occured when I tried to display the snapshot levels some time later
/sbin/snapshot-dual-root -X --target-slice alternate
Snapshot source: internal (/dev/da0)
Snapshot target: external (/dev/da1)
DUMP: WARNING: should use -L when dumping live read-write filesystems!
DUMP: Date of this level 0 dump: Thu Nov 22 11:35:27 2012
cannot open /dev/tty: Device not configured
DUMP: Date of last level 0 dump: the epoch
DUMP: Dumping /dev/da0s1a (/) to standard output
DUMP: mapping (Pass I) [regular files]
DUMP: mapping (Pass II) [directories]
DUMP: estimated 97732 tape blocks.
DUMP: dumping (Pass III) [directories]
DUMP: dumping (Pass IV) [regular files]
.
.
.
DUMP: DUMP: 99077 tape blocks
DUMP: finished in 125 seconds, throughput 792 KBytes/sec
DUMP: level 0 dump on Thu Nov 22 11:35:27 2012
DUMP: DUMP IS DONE
warning: cannot create symbolic link ./etc/localtime->/var/etc/localtime: File exists
warning: cannot create symbolic link ./etc/ssh/ssh_host_key->/config/ssh_host_key: File exists
warning: cannot create symbolic link ./etc/ssh/ssh_host_key.pub->/config/ssh_host_key.pub: File exists
warning: cannot create symbolic link ./etc/ssh/ssh_host_dsa_key->/config/ssh_host_dsa_key: File exists
warning: cannot create symbolic link ./etc/ssh/ssh_host_dsa_key.pub->/config/ssh_host_dsa_key.pub: File exists
warning: cannot create symbolic link ./etc/ssh/ssh_host_rsa_key->/config/ssh_host_rsa_key: File exists
warning: cannot create symbolic link ./etc/ssh/ssh_host_rsa_key.pub->/config/ssh_host_rsa_key.pub: File exists
warning: cannot create symbolic link ./etc/ssh/ssh_host_ecdsa_key->/config/ssh_host_ecdsa_key: File exists
warning: cannot create symbolic link ./etc/ssh/ssh_host_ecdsa_key.pub->/config/ssh_host_ecdsa_key.pub: File exists
/sbin/snapshot-dual-root -X --info internal
Snapshot source: internal (/dev/da0)
Snapshot target: internal (/dev/da0)
1+0 records in
1+0 records out
512 bytes transferred in 0.002590 secs (197688 bytes/sec)
** /dev/da0s1a (NO WRITE)
** Last Mounted on /
** Root file system
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
1450 files, 48306 used, 45284 free (28 frags, 5657 blocks, 0.0% fragmentation)
umount: /tmp/snap-tmp.61556/mnt.61556: not a file system root directory
** /dev/da0s2a
FILE SYSTEM CLEAN; SKIPPING CHECKS
clean, 45285 free (53 frags, 5654 blocks, 0.1% fragmentation)