MAC's use a different file system called HFS+. If the drive is formatted to that file system, then windows will not see it. You could boot a live linux Distro. The latest version of Puppy linux should mount the drive as read only. When you attach the external drive in Puppy you should see a USB icon appear on the desktop. Click once on it and the drive should automatically mount and the File Manager will open.
Your Internal drive in Puppy should be called sda1 or sda2. If you have WIndows 7 it will most likely be sda2. Click once on it and another file manager will open. On the file manager of the external you can select files by left clicking with the Ctrl key or windowing around your files/folders. Once highlighted drag the files over to your internal drive file manager. A small dialog window will open. Select Copy, not Move, and click the Quiet button.
You could also download Ubuntu 12.04. That should see the file system. In Ubuntu you would see a USB icon in the Unity panel on the left. You would need to double click that to mount and open the file manager in Ubuntu. There should be a local disk icon. That is your internal drive. Double clicking on that will mount the drive and open the file manager. You can use the same procedure as Puppy.
If you cannot get the external drive to mount post back. We may be able to mount the external drive manually.