You need software like MacDrive to have Windows7 understand how to write to an OSX drive. Win7 uses NTFS format and OSX uses HFS+. Think of it as Win7 speaking German while OSX is French. You need a translator and that's what a program like MacDrive does for Win7.
OSX can read NTFS but can't write to it. I use Tuxera NTFS to provide the 'translation' to allow that to happen.