External hard disk help

well im using laptop with xp and sp2. i got 40gb on it. i have acquired 2x30gb laptop harddisks with usb cable to connect it to my laptop and use it for extra memory. i have previously done this and worked fine.
my prob is that with the 2 disks i got now… when i connect them to the laptop… it doesnt even recognise them. the green indicator light on the disk comes on upon connection but i cant get my laptop to read them… basically i wanna reformat them and use em for storage space.
pls help me… what shud i do. i got limited time to make em work otherwise ill have to return em