Are you wanting to connect a USB device to to the USB on the back of the router and have the FTP data stored on it?
Precisely! I found out (the hard way) that it can't be done on a
stock 635 AND that it's processor's lack of support for Linux prevents compatibility with common alternative firmware. So, after googling high and low, I thought I'd ask a question on the home forums for any (be it unorthodox) way to fulfill this wild dream of mine

.
BTW, what OS is the 635 running?