time machine troubles.

I recently purchased a lacie 320gb external hard drive from my local apple retail store. Once i got home i configured it to work with time machine, it worked great the first time but then i reinstalled leopard and now when i try to back my macbook it takes ten minutes or more (still waiting) just to prepare the back-up.

is this normal (didn't take this long before the reinstall) ?

if not what do i do?

i am hesitant to do another reinstall but will if it is needed.

thanks in advance