How can i play portable hard disk on my ps3?

May be...

i have a western digital hard disk 160 gb but when i connect it to the ps3 via USB it won't recognize it what shall i do?

You need to format the hard drive first of all. If it is brand new, you need to hook it up with your computer and make it's format FAT32. If it is not FAT32 then the Ps3 will not recognize it. Then make 4 folders in the hard drive labeled Picture, Music, Game, and Video. For more detailed instructions and how to make the hard drive FAT32 go to:

http://www.wikihow.com/Add-an-External-H...