Try running without the external drive being connected for a day or two and see if the beach ball effect goes away.
There is a possibility that the LaCie is not functioning as designed and your system is checking, every so often, to see what is on the Firewire/usb bus. If the drive has dropped off, then the system will spend some time trying to locate it, at the expense of other applications.
Since upgrading to Snow Leopard, In have been having trouble with the external drive disappearing. This means that SuperDuper cannot backup etc. I am running a MacBook Pro 15". The external drive is a La Cie with 1TB capacity. Reboots do not help. Disconnecting external and rebooting, then connection can solve the problem once in awhile. Could this be a Snow Leopard problem? I am also getting a lot of hanging with the infamous "beach ball" on both Omniweb, and Safari. Any suggestions would be greatly appreciated of course.