This is a known incompatibility between ProTools 5.1 (and 5.1.1) and the DVD-RAM Tuneup extenstion. Disabling this extension helps.

- From the Apple menu, choose the Control Panels pop-up and select "Extensions Manager"
- Scroll through the list and locate "DVD-RAM" Tuneup. Click the "x" in the box next to this extension to turn if off.
- Restart your computer/

* If you need this extension, it is recommended that you create an separate extension set without Digidesign extension on to use DVD-RAM tuneup.