What model of drive is it? A3000 usually have Chinon FB-354. It's common for the disk detect switch in these to develop intermittent contact issues that causes this problem, and also warnings that the disk is write protected when it isn't.
While replacement switches generally aren't available, you can open the switch to access the internal contacts to clean them.
Unsolder and remove the LED which allows the plastic frame around the switch to be removed. Then heat the 3 legs of the switch, which lets the entire switch slide off the edge of the PCB. Use a small craft knife to carefully release the retaining clips holding the switch together. Be especially careful not to lose the small springs inside when the switch comes apart. Use a Q-tip or similar with isopropyl alcohol to clean the gold contact surfaces. Reassemble the switch and solder it and the LED back onto the PCB.
It's a delicate job, so if you have the finesse of an outraged gorilla, you might not want to try this as you'll probably end up with no switch and an unusable drive. You can try squirting some isopropyl alcohol down the side of the two switch actuators to flood the switch internally. You can use the drive immediately afterwards, the alcohol gradually evaporates. Sometimes that works, but often the intermittent problems soon return.
Another idea is to buy a broken drive of the same model and use the switch from that. Though you won't know the condition of the switch until you try it.
