Hard drive can't be read since upgrade to Leopard

I recently upgraded my emac to leopard and the external hard drive which i was using to store all my images and files can now no longer be read. My old ibook running 10.3 can still see and interact with the HD but the emac says it is unreadable and offers to reformat it. I also recently got a nice used macbook running leopard(the reason i upgraded the emac in the first place) and it doesn't read the external either. So why isn't leopard seeing the info on the hard drive? Is there some setting that needs changing on the computer? Reformatting the drive isn't an option since all those files on it I need. Any help appreciated. p