Well when you partion a drive in Boot Camp (Mac), you can read the Windows drive from Mac, but not write to it. When you boot to Windows, you can't see it at all.
That sounds similar. I don't think Windows can read the Mac filesystem (which is a Unix variant, but I forget the name). There may be a Windows program you can use to overcome that.
You may be able to partition the My Book and format the different partitions separately with the different operating systems.
QED, just sayin'