Slow Transfer Between External Drives
A method I use to move large numbers of files is the Terminal mv or cp commands. The first command actually moves directories/files from one location to the other, the other just copies the directories/files. This is much quicker than Finder, which adds a tremendous amount of overhead to the process.
Secondly I strongly suggest you go to System Preferences &gt; Spotlight, and add both external drives into the Privacy pane. This stops Spotlight from indexing the files. The drives can be removed from the pane when you're done if you want these drives indexed.