initiate file transfer between two devices from third machine

I'm wondering if someone could help me; I'm about two transfer about
400GB of data from a USB external HD to my time capsule. I figure the
easiest way to do this would be to just plug the usb drive directly
into the usb port on the time capsule and initiate the transfer from a
third machine--which happens to be connected via wifi. I'm wondering
whether or not the files would first go through the initiating machine
(wifi macbook) instead of directly from HD to time capsule. If so, is
there any other way I can get the files to take a direct route?
Obviously, wifi is considerably slower than usb 2.0's 480mbps. Any
help would be appreciated--thanks.

I'm not sure i am going to be able to help to greatly but when browsing through the setting of TC there seemed to be a way to do that. I know in the manual setup you can transfer files from TC to an external HD but i'm not sure it goes the other way. Look through the manual settings a bit and see. Sorry for the crappy reply.