If they're anything like the LaCie D2 External Hard drives, then it'll be a doddle. I've replaced the HDDs in 2 of the D2 enclosures and it really is a piece of cake. I doubt the Porche ones will be any different.
The D2's basically disassemble as follows:
1. Unscrew 2 screws at the back to release the back plate (where the USB/Firewire ports are etc)
2. Slide the casing off, revealing a 'cage' containing the drive along with the circuitry
3. Release the IDE Cable and Power Cable from the HDD
4. Unscrew 3 screws on the bottom holding the HDD to the cage
5. Slide out the HDD.
6. Reverse process for installing the new drive.
LaCie's seem to be very easy to disassemble, so I doubt you'll have any problems
(btw, the reason I have actually changed the HDDs on the 2 LaCie drives in the first place is to remove the Maxtor drives they shipped with back then to replace them with Western Digital drives)