no boot with USB drives

I have vista home basic (new instalation not a XP upgrade) installed on a 2008 computer and want to use a maxtor 3200 USB external HD (2006 model) for backing up files?
However when I boot up the computer with the drive connected an error mesage (NTLDR is missing. Press ctrl+Alt+Del to restart) occures and the computer will not boot even when ctrl+Alt+Del is pushed. The drive has to be removed before the computer will reboot.
Another problem occures when I boot up with a USB flash drive conected. The computer will not boot into windows and will not give me any error messages just a flashing cursor. Once again the drive has to be removed before the computer boots up.
Any suggestions of how to solve these problems
Thanks