Sounds like you need to change the boot order in your BIOS. If the computer is looking to the external to book, go into the BIOS and change the boot order so it looks to an internal HDD first.
I have a HP Pavilion a824n. I've upgraded my hard drive from a 200gig to 1tb.
After I transfered all the existing info to the new drive, I formatted the old drive and put it in an external case. New drive is working perfectly.
Unfortunately when I boot the computer, the computer somehow thinks the old 200g is still my boot drive and is trying to use it to boot. It is giving me the error "NTLDR not found..."
I've done some searching and found out that this is caused by NTDetect wanting to boot off the 200g drive, but I can't find out how to fix it.
My temporary solution is to leave the external hard drive powered down when I boot the computer, and then power it up after Windows is loaded. It functions perfectly normally when I do this, however this is a clunky and annoying way to do things.
Any help or suggestions would be appreciated.