(redirected) External USB HDD boot

I own a Latitude D420 BIOS version A03, POST boot order is USB, DVD drive then internal HDD. I currently have windows 7 running on my 80GB internal HDD and have been trying for some time now to get my WD Black external to boot Lubunu so that it would host VirtualBox and GNS3 to help me out with my education. The other relevant specs are: Core 2 Duo with 2GB RAM free (+512 system RAM) so it would be a bit tight for all the virtualization but should work, please don't tell me to buy a new puter :P Anyway that has little to do with my being able to even boot from the USB external HDD. I have set BIOS to look at USB first and there is GRUB on the external, which I checked and repaired. When I use a flash drive to boot from, all goes well, no problems, but when trying to boot the spinning platter hard drive its a no go. I've run "Boot-Repair-Disk", an Ubuntu utility used to repair any Linux boot problems or misconfigs. Any ideas would be welcome. Thanks in advance T.V.