I'm not very familiar with the Mac OS. However, I do know that NTFS is the preferred file system in Windows. Later versions of Mac OS X support read-only access to NTFS but not write access. This makes me think the drive is currently formatted NTFS. If this is the case, you'd need to format it using a different file system to which both Windows and the Mac OS have write access. Formatting the drive will erase everything on it, so you'd need to back up the data on the drive first.