PC-Mac Backup Issue with ext HD.

Hello, so I switched to a mac years ago, and my parents finally decided to do the same. They wanted their pictures and documents backed up and put on their new mac. I have an external WD mybook mirror which i tried plugging into the pc, with the drivers installed the disk was found, but didn't mount. I did some reading and realized that you need software for the pc to mount and be able to open the mac formatted had drive. So I installed the software, an opened up the HD. Then I copied the stuff they wanted backed up onto the HD. Well... Now it won't mount back onto my mac, and when I run the disk utility and try to verify or repair the hard drive I get errors. So I think I understand how I messed up. I wrote information in the wrong format onto the drive and now basically it's corrupt. I can still mount and open up the hard drive using the pc, but I am hoping there is some way of repairing the drive and avoiding having to dump and reformat it. I really over estimated the pc software for use with a mac hard drive and I am full of regret... Now gigs of important backed up info hang in limbo... Any help is much appreciated.

Thanks in advance ~ Johnny