- In a Quadra 900/950 or IIfx , the Serial Switch Control Panel must be installed in the System and set to compatible. - In a IIfx, the Cache Switch Control Panel being used is not the standard Apple issue and it is set to function in the Quadra mode. If so, set it to the IIfx mode or use the Apple issued 040 Control Panel and set it to Faster.- OMS Environment is corrupted. Restart Mac without the OMS Extension loaded and try to relaunch PT. If this works, throw away the OMS Extension and OMS Preferences, and reinstall OMS from the original installer disk.- Corrupted Operating System. Reinstall from original disk(s).- SCSI Bus not terminated properly (IIfx = black terminator).- Bad RAM in the computer.