Try this. Put the seagate disk in your mbp. Startup with the leopard cd (holding down the "c" key of course) Now when you get to the installer screen, go to the top menu and select disk utility from the "utilities" menubar. When disk utility loads, see if it can find your Seagate disk. If it does, follow these steps as outlined in the apple knowledge base article found here:
http://support.apple.com/kb/TS1550?viewlocale=en_US
1. Open Disk Utility.
2. Select the disk's icon on the left side of the window, which usually appears with a numerical capacity and is offset slightly to the left of any volume icons. If you see a tab named "Partition" appear, proceed to step 3.
3. Click the Partition tab. (If you only see tabs named First Aid, Erase, RAID, Restore, then you have selected a volume on the disk instead of the disk itself--repeat step 2.)
4. From the Volume Scheme pop-up menu, choose the desired number of partitions (or 1). (choose 1 if you just want the hard drive formatted as one single drive)
5. Click the "Options..." button.
6. Select a new partition scheme:
* Use Apple Partition Map partition scheme if the disk will be used with a PowerPC-based Mac.
* Use GUID partition scheme if the disk will be used with a Intel-based Mac. (you want this one if you have a new unibody macbook pro or intel based cpu)
7. Click OK.
8. Click Apply. This will erase the disk.
Now Leopard should be able to see your disk for installation.
