I just got finished dumping XP off of one drive for the 32bit RC with Vista on the latest Virtual PC beta there. The reformat to ntfs would normally make any external hard drive or usb flash drive noticed right off once Windows XP, Vista, or 7 detected it. You may want to look in the device manager under disk drives as well as looking in the Disk Management tool to see what drives appear there.
Instead of trying to boot up with one those already plugged in shutdown, unplug either of the flash drives, and then wait until 7 is up at the desktop to plug it in there. That should immediately see a popup indicating the drive is then detected.