increase harddisk size....
 

i have a machine, which is currently using /dev/sda with 76G size. but, then i plugged in mount hard drive which give me another 138G size under /dev/sdb. how can i mount to the /dev/sdb? i believe /dev/sdb is a fresh hd. which i havent doing the formatting. please advise. thanks

What distro are you using? If you have a GUI, then either QTParted or GParted will give you a pretty front-end to partition editing. Google for more info on either of those.

If you're only storing data there (movies, music, etc..) then just create one big partition - either Ext3 or ReiserFS.