In some cases, trashing the contents of Digidesign Database folders will fix this. Follow these steps:

1. Quit ProTools and all applications
2. Locate and open the Digidesign Database folder at the root level of all mounted hard drives.
3. Drag all the contents of each Digidesign Database folder to the trash.
4. In the Macintosh HD, go to Library > Application Support > Digidesign > Databases > Volumes
5. Drag all the contents of the Volumes folder to the trash.
6. Empty the trash.
7. Use the Disk Utility to verify and repair disk permissions:
Located in Applications > Utilities, choose Disk Utility. In Disk Utility, choose the Macintosh HD and select "Verify Disk Permissions." Once complete, click "Repair Disk Permission."