Motherboard & external hard drive problem

I recently bought a Verbatim eSATA & USB 2.0 1,5 TB external hardrive.The manual tells me to connect the drive to my pc when my pc is off.i connect it with the eSATA cable provided and i do everything right but when i turn on my pc it gets stuck for a while in the screen where it says about pressing DEL for setup etc and after that it tells me that it cannot boot from the drive and tells me to restart the computer.But when i connect the drive through the usb connection everything works fine.I believe that when i connect the drive in the eSATA the motherboard ignores my main drive and thinks that the main drive is the external one i tried to get it to boot from the main drive of my pc by pressing F8 for boot device sellecting but it only shows me the cd drive as a boot device.
Thanks in advance for the help!!!

If you can go into BIOS you can probably select another boot order. If you have already tried this, then I am not sure what to suggest.
Are you using windows? (Rather than Linux)
Someone else will probably know more.

I tried that but when the extternal drive is connected as boot devices it shows only the cd drive and I tried into 2 os windows xp and windows 7

It might be a driver problem. If you could try to manually reinstall them that would be at least a place to start.

Trying to incorporate external equipment into your computer is always difficult because it is not really (ever) part of the computer.