Trouble with external hard drive?

I have a HD-CE500U2 Buffalo external hard drive. Up until 10:00 am this morning's it's been working fine in conjunction with my regular old Macbook. This morning I connected it and it is not recognized (aka it doesn't even show up). I see the drive whirring away and I know it connects perfectly to my X-Box and is totally readable. What's the problem and how can I solve it?