My macbook won't recognize new 320gb external hdd

[blakebone]

(Whoops i made a mistake in the title i ment to put INTERNAL hdd sorry) I just bought a new mobile hdd for my macbook and when i put it in then i put in the mac os x start up disk and it says select the hdd you want to download the software onto and it won't detect the new hdd so i had to put this old one back in. so can someone please help me with this problem? thanks

 

[blodwyn]

(Whoops i made a mistake in the title i ment to put INTERNAL hdd sorry) I just bought a new mobile hdd for my macbook and when i put it in then i put in the mac os x start up disk and it says select the hdd you want to download the software onto and it won't detect the new hdd so i had to put this old one back in. so can someone please help me with this problem? thanks

I would re-install the drive and then boot the DVD and format the drive using Disk Utility before you start the install. Or put it in an enclosure temporarily and format the drive before you install it.