Take it back
Take it back!
I bought a FreeAgent 320GB a year ago and it is garbage ... especially on my Mac.
It goes to "sleep" after 15 minutes and it is difficult to wake. Sometimes you can remount it using Disk Utility and sometimes you end up formatting it.
Under Windows, you can download a utlity to change the delay in which it goes to sleep. But even there, every once in a while when you reboot it can't find the drive and you have to start go through utilities to get it back.
My advice, if you can still return your drive ... take it back.