i connect external harddrive (ntfs format)
Then the answer to the second question in post #2 is "no, it is not a valid ext2 file system". It is ntfs. Mount it as ntfs.
As masonm said, remove the line you added to fstab. Then mount manually with the following:
ntfs-3g /dev/sdb1 /mnt/Mandriva
(If you do not have ntfs-3g installed then "mount -t ntfs" can be substituted)
And I bet that will work. When it does, you can edit /etc/fstab to add a line that says:
/dev/sdb1 /mnt/Mandriva ntfs-3g defaults,noauto 0 0
(If you do not have ntfs-3g installed then "ntfs" can be substituted)
It's checking at all, instead of just failing the mount, because you put the last field in fstab to 2.
You don't really want it to try mounting at boot, you really want it to mount when it is available... otherwise you'll get an error if you try to boot when it is not plugged in.