Use of off-disk temp files

As I sit here listening to my massive external USB drive ramp back and forth across it's surface, I was wondering if there would be a desirable trade off, wear against speed (especially with the new USB 3 drives), with usigng 30 or 40 GB of a (selectable) host drive to move stuff off of the target disk to to make room to move stuff around.

This would assume two things:

1. You'v got space to spare on the host systems internal non-usb drive(s) (I've had problems with some USB chip sets writing from USB to USB)

2. That free space is at least 2 to 3 times larger than the largest single file on the disk being defragged. (I have some 10gb Video Files)

Thoughts anyone?

I suppose I could try this manually using Beyond Compare, but there is no easy way for me to read which file is where positionally on the drive.