The only disk formats the Mac OS can write to are HFS+ also known as Mac OS Extended and FAT/FAT32. Because Vista can only be installed to an NTFS format drive or partition, you cannot have read/write access to it from Mac OS. If you had XP, and had the drive formatted FAT, which would also limit it to a size of less than 32 GB, you would then be able to read/write from either operating system.