My /root filesystem get fulled up .I want to extend root partition using extended secondary harddisk
I assume your root / partition is sitting on one of the primary partitions: the first three (3) partitions, usually sda1, sda2, and sda3.
You may extend root partition provided these requisites are met under assumed case:
--that there are LESS than Four (4) primary partitions existing including the extended partition.
--else, if there are Four (4) primary partitions, STILL there is remaining space to accommodate expansion of one partition;
Other than that you have to kill (delete) one primary partition to accommodate expansion, or shrink the extended partition (counted as primary) to open up a space.
If your root partition is a logical partition in the extended, you only need to shrink some logical partitions to accommodate needed space.
If possible do not use console disk shrinker you can easily mistake and lose important data: use gparted or any other graphic shrinker.
In all things: BACK UP FIRST.
Hope that helps.