here is the output for that switch
> show system storage partitions
fpc0:
--------------------------------------------------------------------------
Boot Media: internal (da0)
Active Partition: da0s2a
Backup Partition: da0s1a
Currently booted from: backup (da0s1a)
Partitions information:
Partition Size Mountpoint
s1a 183M /
s1a 183M /
s2a 184M altroot
s3d 369M /var/tmp
s3e 123M /var
s4d 62M /config
{master:0}
> request system snapshot slice alternate
fpc0:
--------------------------------------------------------------------------
Formatting alternate root (/dev/da0s2a)...
error: Could not format alternate root