Critically important for Davis users- If you are either upgrading to Snow Leopard, OR installing the Davis drivers from either Davis or Silabs, then installing WD Mac after, you MUST repair permissions on the hard drive BEFORE installing WD Mac. One of the 3 combinations mentioned here is breaking the permissions on the /usr/lib/ folder and not allowing us to install the correct lib files. Then after repairing disk permissions, install WD Mac and proceed to set up normally. It's ok if you already installed WD first and then came here and read this, you can reinstall again after performing the steps above.
To repair permissions - Go to Utilities folder, open Disk Utility, select only your MacIntosh HD (the system hard disk) and click on Repair Permissions. Should take about 10 minutes and you will likely get a ton of messages in the window. (Would be nice to know which of the drivers above is responsible so wecan inform the vendor) or if it's a Snow related breakage, it will just say repairing /usr/lib/ and at least we will know.
This should be performed before AND after any system upgrade as good system maintenance, and after installing third party drivers such as mentioned above.
All the best
Jmar