my powerbook G4 is having a problem!! i tried the apple hardware test and it detects an error on the mass storage. i reinstall the MAC OS and when the installation is complete and restart the computer. the computer stays at the blank gray page with a picture of apple in the middle of it..and it won't start up the system. does that mean it's time for me to get a new harddrive? how much does it cost to replace a harddrive at the apple store?