I have the feeling that it should *not* take almost a day to move 50 GB from one external HD to another.
-brotherS
I wonder if transferring from one external to another is killing throughput. If you have space on a local drive to store the data temporarily, it might actually be faster to copy from one external to the local drive then copy from the local to the other external.
Also, if you have either or both of these drives connected via a hub, move it/them to ports on the PC itself.
I wouldn't be surprised if USB drivers/controllers/hubs aren't very good at dealing with hi-speed throughput to multiple devices concurrently.
Remember to use a good copy utility (robocopy or such) that knows how to do copies with large buffers and overlapped I/O so the device drivers have the best chance of getting things optimized.
However, shoving 50GB of data around will still take some time - I'd guess that an hour is the best possible time you'd get, and that's with all components being top-notch.