If you have used a Western Digital Firewire hard drive with your Mac, and have upgraded to Snow Leopard, you may run across an error where the "IOFireWireWDHID.kext" extension will not load. This may happen immediately after updating, or may happen when you update other software such as iTunes, in the case of one user.
CNET Forums member "ndellomo1" writes:
"After installing iTunes 9 using Software Update on an iMac 5,1 (2 GHz core 2 duo, 2 GB 667 MHz SDram, Mac OS X 10.6) I received a message that IOFireWireWDHID.kext was not properly installed. This extension was dated 2006. Is it required? If so, how do I properly install it?"
This problem happens because the kernel extension is not compatible with Snow Leopard and needs to be updated by Western Digital. It will sit in the extensions folder until some application tries accessing it, in which case the system will fail to load it and result in this error.
Until Western Digital addresses this problem, you should be able to remove the extension from your /Macintosh HD/System/Library/Extensions/ folder in order to avoid warning messages about it. The drive's basic functions as an external storage device should still work with Apple's built-in Firewire drivers, but you may lose some features specific to the drive (i.e., customized performance options) until the drivers are updated.
Similar problems to this may crop up with other third-party kernel extensions, and usually the problem is the same thing: an incompatibility with Snow Leopard. This may happen if your computer is booted into 64-bit mode in which case rebooting to 32-bit mode should help, but also could just be that the driver needs to be updated.