V1-V3 PS2's only play -R's, and only sometimes.
V4's ... some of them play +R's (the late-model V4's that came in the Gran Turismo bundle play +R).
V5-V10 should play everything.
I'm guessing you have a V1, V2, or V3.
If the PS2 won't play -R, then it won't play +R either. If it plays +R, it ought to play -R. See how this works?
YOUR PS2 is just useless for backups, sorry.