From Pete Bankert:
He says you should use the disc repair service on your computer to check out whats wrong.
OR delete and reinstall!

OK, please do not reply with "Ive already done this"
Please follow the directions carefully:

On eac and every drive, at the root directory is a file call "Digidesign Database" delte them all, and empty the trash.

Now you didnt state whether you were using PTHD or PTLE, so this will be tough for me:

In your "home" directory, which is just below the desktop icon in an open folder, it might be called by your proper name...... go to Library/Preferences......

delete "DigisetupOSX"
delete com.digidesign.ProTools HD or LE.plist
delete Protools XXXX.preferences
delete Pro Tools HD or LE v7.X Preferences

anything else in the preferences folder that has DIGIDESIGN or PROTOOLS or COREAUDIO in the line, look carefully.

EMPTY the trash.

REBOOT

Start a new session, if that works without errors, then open the old session, if the old sessions cause an error, they are corrupt, youll need to save your audiofiles and delete the corrupt sessions, and redo the above.

Have fun

Oh, yeah, one more thing.......... in an open Protoos session, open the workspace window, and make sure your drive is set to write, NOT read, NOT transfer. Also make sure you are not recording to your boot drive!!!.

Thanks TVPostSound,
Ive followed your instructions and it worked, everything is back to normal.
What exactly is a corrupt file/session? How does it happen. Can I expect this to be a ongoing problem? How do I avoid this?
Once again thanks for your help!
Shindog.

Glad to be of service!!!

Sounds like the session wasnt corrupt.

I had you do all the prefs just for the sake of time.
Dont forget to reset all your preferences in Protools now!!!

The Digidesign Databases seem to get corrupt often.
Next time it happens, try only trashing the Digidesign Databases on all the drives, they get rebuilt when you open Protools.