If you make a partition on it with a FAT32 file system, both the Mac and the PC will be able to read it and write to it.
Is there an external hard drive that will allow moving documents between a Mac and PC? Everytime I buy one that says works with Mac and PC, it turns out that it can only read and write on one, and the other will "format" it to use. In other words, have to erase everything stored on it and start over to switch over and use exclusively on either Mac or PC. Anything that can go between the two?