Forget the software that came with it, format the drive as Mac OS Extended (Journaled). Connect it to your iMac, open Disk Utility and format the drive. If you need a drive for Windows 7 EHD then get another drive. Sharing a drive with Windows and OS X is a losing proposition paticularly if it's a backup drive. The reason it's not a good idea is it's full of compromises such as file sizes, partitioning issues, reliability etc.