Won't accept the disk and grinds - Sony Playstation 3 (PS3) Console

The first is that "Disc Read Errors Diagnosis mode could be the solution.

Load your PS2 without a disc inside and wait until you get to the main menu.

Press Triangle and highlight "Console", then press Triangle again and set "Diagnosis" to ON. What does this setting do? Basically, it focuses the laser on the disc so that it has an easier time reading discs.
In addition, you must turn this setting on EVERY time you load your PS2, as the PS2's system settings are never saved when you turn off your console