Is there a way to use My Book Essential on Windows 7 after on OS X without reformatting?

Hi. I just Boot Camped Windows 7 on my iMac, and I need an external hard drive to transfer files back and forth to each operating system. This seems like it’s not going to work so far. Is there any way around this, or should I buy another external hard drive that I can move files from the one I already have onto it and vice versa? 

You could format it in FAT 32 on your Mac, but you will have a tendency to corrupt the data going from Windows to Mac.  You might be better off just getting another drive, one for each.