Mac hard drives should be formatted as "Mac OS Extended" (aka HFS+), but Macs can also read/write to drives formatted as FAT32.
Windows hard drives are usually formatted as NTFS or FAT32. A Windows system cannot read or write to Mac formatted hard drives without software like MacDrive.
You can download and install a NTFS utility on your Mac that will allow it to read and write to NTFS formatted drives.
-DH