I'd assume that the problem is that the USB drivers are missing in initrd. SUSE does not seem to consider a USB drive as something to boot from. During installation, all drivers are loaded, but not included into initrd, so the reboot fails.
My suggestion: try to boot from CD. You should either get the option to boot from HDD directly (if the USB drive is detected), or you can enter the manual installation mode and load the drivers yourself (usbcore, sd-mod, usbstorage, ehci-hdc). Now you should be able to boot the installed system. After that, you need to run mkinitrd -m <the usb modules>. This is purely theoretical, since I have never attempted it myself.
An alternative option could be to boot a LiveCD (e.g. KNOPPIX), mount the USB drive's root partition, chroot it and run mkinitrd from there.
Hope you it helps!
(Please report if it does)