I think the problem is incompatibility with ExFAT on Mac OS X. Is there no way you can put that drive on a network such that both your Mac and Windows systems can access it?
We rely on file systems working correctly. If the OS is incompatible or just is inconsistent with a particular file system then PM will have problems.
I suggest finding a way to make the drive networked or format the drive in a Mac OS X native format like HFS+.
-Kirk