What's the best way to back up an external hard drive onto a Macbook?
You can use Disk Utility to create an image of the external hard drive.

Plug in the external hard drive and then start Disk Utility--it's in the Utilities folder under Applications--and then select the external hard drive from the list along the lefthand side of the Disk Utility window.  Once you've selected the external drive, click on New Image at the top of the window and it will ask you where you want to store the image file.  Select a location and then let it do its thing.

Once completed, you can restore the image to the same or another external drive or you can simply double-click the image file to mount the disk and explore its contents using finder as you would normally.