WD passport won't show up on Imac

I wasn't sure where to put this question, so I am sorry if it needs to be somewhere else. I have a WD passport USB external hardrive hooked to a computer at work that uses Windows 2000. In the past I have been able to just hook it to my Imac it shows up and I am on my way. Recently I can't get it to show up. It also will not show up on a computer that uses windows XP. Any ideas what may have happened and how I can make it show up on my Imac? It still works on Windows 2000.
Thanks,
Ed