Mac external Disc is wiped when i connect to XP PC

I have an old imac which i had an external hard disc connected to it . It was partitioned to store music on one disc and photos on the other . My imac broke down and i connected the external drive onto my XP laptop via firewire into pcmcia card which didnt show and i then used a usb connection to view the photos on the hard disc . I couldnt open the disc. I have now connected the disc to my macbook which used to be able to look at the photos previously and the disc shows all free space on the photo partition ( am new to mac as you can see ) how do i retreive this data and what have i done wrong here thanks jk