Windows Vista Home Premium on a Dell XPS M1530 laptop.

The title basically says it all. Whilst using an external USB hard
drive, Process Explorer gives the CPU useage in the 90% -plus range.

It's a dual core system, and it appears to only be using one core at
full useage, as the temps are in the high 50's and low 60's (celcius),
and full useage of both cores typically puts the temps into the 70's.

What's the reason for this? Indexing? Does the CPU need to be doing
this, and is there a way to stop it? I use the external hard drive for
videos, music and photos so the contents are not being used permanently.
is there a way to stop this CPU useage, and if so, would it affect
performance much?