Font First Aid attempts to open every file on the disk using the Resource Manager (a bult-in part of Mac OS), to check the file's resource fork. This error message is displayed when it encounters a file it cannot check. Usually this error message indicates it encountered a bad resource map or a file that was open and had restricted access.
If a file has a bad resource map, an error will be encountered when another application attempts to open it with the Resource Mananger as well.
If you have a Sound Track enabled (such as Platinum Sounds), this error message will display as that file is open and has restricted access. (To verify: open the Appearance control panel, change the sound track to None and run Font First Aid again.)
When this error message is displayed click Continue. Font First Aid will scan the rest of the volume, fixing all the remaining font faults. Only one scan should be required, even if the message above is encountered.
For more information, please see the following Tech Info Library article:
Article 60444: "
Font Manager Update 1.0: Read Me"