The data you write to the harddrive isn't written in a raw format, it is indexed and broken into pieces. This system of organizing is known as a filesystem but it is seperate from the physical disk. Think of it as a way of writing data. So there are many different filesystems.
This link might help
There are plenty of fiesystems. FAT32 can be read/written to by most operating systems. Linux doesn't have one default filesystem but generally ext3 is considered the standard one, with others such as ReiserFS, XFS and JFS being available. Being that the filesystem is just a matter of how the data is written any filesystem can be used on any harddisk - one is not a part of the other.