Mac Partition on external hard disk not appearing

I have a 500GB external hard disk, and I've partitioned 250GB in HFS+ for my MacBook Pro and the other half in FAT32 for my families windows machines.

I've had this hard drive since September 2011 and have been backing up my machine without any problem until today, when i went to plug in the drive the windows partition appears but my Mac partition never appears.

The hard drive hasn't been dropped or abused in any way.
could it be something with my mac? (i don't think it is personally)
any advice would be greatly appreciated.

as a precaution i've copied all my data from my windows partition onto my macs hard drive.