[CentOS] Disappearing directory

Hi All and happy new year,

Via a cron job a USB disk is mounted on a Centos 6.4 machine for backup 
and dismounted after. I've noticed this failing. See below, the backup 
directory that was in /mnt had disappeared, so creating it again as 
follows...

# cd /mnt
# mkdir backup
# mount /dev/sdb1 /mnt/backup/
# ls
backup
# cd backup/
# ls
boot.tar.gz  etc  etc.tar.gz  home  home.tar.gz  ident.txt  lost+found  
named.tar.gz  root.tar.gz  usr.tar.gz  var  var.tar.gz
# cd ..
# ls
backup
# umount /dev/sdb1
# ls

# cat /etc/redhat-release
CentOS release 6.4 (Final)

After the umount the directory vanishes. What gives or have I lost the 
plot in 2014. (This works as I'd expect on an old fc14 system I have)

:-) Ken




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