If you want that both the mac and the pc are able to read and write each others files, you should make one or more FAT32 partitions on the drive.
If it's OK to reserve (say) half for use on the PC and the other half for use on the Mac, and it's NOT necessary that they can access the files of the other machine (not even for transferring files between PC and Mac) you could make a Mac formatted partition (for use by the Mac) and a NTFS partition (for use by the PC).
Of course, you can combine it also: 1 Mac-partition for the Mac, 1 NTFS partition for the PC and 1 FAT32 partition that both can use. It's totally your choice depending on how you plan to use that disk.
One note: external hard disks shouldn't be used to STORE data. They aren't reliable enough so sooner or later you'll lose everything you didn't store elsewhere also. An external disk is (only) good for backing up data from the internal hard disk of your Mac and PC (that's a copy, just in case something happens with the original) and to transfer files between machines that aren't networked. But NOT as the only place to store data. Of course, you can buy the music again, if it's lost, but that's more difficult with your own pictures.
Kees
Kees
I just bought a WD My Book Essential 3TB hard drive and want to use on both my mac book pro (mac os x lion) and my pc laptop (windows 7). I mainly want to use it to store pictures and music. I understand I have to reformat it for the mac, but I am having problems seeing the files on the pc. Any suggestions???

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