If by “blank disks” you mean burned discs, then the problem is that the laser has “worn out”.
The problem you described is not an issue with the motor; it is the laser. An optical drive uses a laser to read and write discs and after enough usage, to put it simply, the laser becomes weak. Therefore, when the weak light emitted, it does not reflect back nice and clearly and with full power as before. Because the pits and lands of a pressed disc are much more distinctive and clear than those of burned discs, a drive with a worn down laser will still be able to read pressed discs but not burned discs—and eventually will not be able to read even pressed discs.
As Mark said, your best bet is to try to adjust the gain of the laser to increase its power. Usually, when an optical drive is manufactured and calibrated, its laser will not be maxed out, so there will still be room to “turn it up”. Try the article he linked to; hopefully that will do the trick.
> It won't read any blank disks… What's strange is that it DOES read original disks, but not blank/written disks How exactly is it supposed to read a blank disk? – Synetech – 2011-12-05T00:44:06.520