Extyernal disk is not seen, but recognized.

I got a new 4200+ HP, AMD 64 dual core, desktop computer and want to use it
with the external
80 GB HD which I have used before with a custom built CPU which also uses
windows xp, same as the present, new HP CPU.
When I boot up with the HP new CPU and have the external disk connected, the
bootup takes forever to get to the desktop. It stops and continues to sit
there BUT immediately boots to desktop once I
disconnect the external disk.( goes tyo an USP input)

The same is true for shutdown. It just sits there and does not shut down
until I disconnect the external disk, then shutdown is immediate.

Another thing. Once I start up (and have the external disk disconnected to
get to the desktop) and then connect it, I cannot see the external HD under
Explore.But I get the sound that it found a new device.

I have to go to the Administrative tools, then Diskmanagement and let it
find
the external disk. Once it does. I can see it in the Explore position.

What causes this strange behavior? The external enclosure is fine. I took it
to the computer shop, where I bought it months ago and they even connected
it to their running computer and it was immediately seen.But at home it acts
this way whether I try to use the external drive with a laptop or the new HP
desktop.

I am totally puzzled at this time.