Leave the 500 GB unformatted. Here's why: If you pre-format it FAT-32 from OS X, Windows 7 will refuse to install. And you can't pre-format it NTFS until you start the Windows 7 installer. Linux Ubuntu requires an Ext2 or 3 format before it will install and again you can't do it from OS X. Yes, you could probably use a third party Linux formatting tool but that may get you in trouble. Better to wait later on when you decide to install Ubuntu and use their built in partitioning and formatting utility.