Is the external drive always powered on, or does it have to come up to speed when you boot the MBP? If it's powered on, but gone into a sleep mode itself, it may not be waking up when the USB line becomes active on the MBP. So, it would work if you booted and then plugged in, but maybe not if you booted with it plugged in. I've had the problem with a Seagate FreeAgent drive that spins down when not accessed in a while. I get the beachball when Finder needs it. Eventually it wakes up and responds, but it can be as much as 10-15 seconds of beachball first.