OK. I did not understand that correctly. My suggestion is that you create a new partition on the Buffalo drive large eenough to hold all the data on your internal drive. Then clone the internal drive to that new partition.
To resize the drive do the following:
1. Open Disk Utility and select the drive entry (mfgr.'s ID and size) from the left side list.
2. Click on the Partition tab in the DU main window. You should see the graphical sizing window showing the existing partitions. A portion may appear as a blue rectangle representing the used space on a partition.
3. In the lower right corner of the sizing rectangle for each partition is a resizing gadget. Select it with the mouse and move the bottom of the rectangle upwards until you have reduced the existing partition enough to create the desired new volume's size. The space below the resized partition will appear gray. Click on the Apply button and wait until the process has completed. (Note: You can only make a partition smaller in order to create new free space.)
4. Click on the [+] button below the sizing window to add a new partition in the gray space you freed up. Give the new volume a name, if you wish, then click on the Apply button. Wait until the process has completed.
You should now have a new volume on the drive.
Clone using Restore Option of Disk Utility
1. Open Disk Utility in the Utilities folder.
2. Select the destination volume from the left side list.
3. Click on the Restore tab in the DU main window.
4. Select the destination volume from the left side list and drag it to the Destination
entry field.
5. Select the source volume from the left side list and drag it to the Source entry field.
6. Double-check you got it right, then click on the Restore button.
Destination means the new partition on the Buffalo drive. Source means the internal startup drive.
If there is not sufficient space on the Buffalo drive for the needed new partition, then will need to get another external drive or rely on the Time Machine backups that you now have.