PS2 doesn't read DVD's, CD's PS1. PS2 games work.
 

I bought a used PS2 from ebay. The seller says it is chipped, but doesn't know which chip.
Now, PS2 games work fine. However it always gets an Error reading CD/DVD when I insert a PS1 game, CD or Movie DVD.
However I don't have Backups of PS2 games to test if it reads these. Or if the Chip works.

What's wrong?
If its of any help: It's a SCPH 30004R Model according to the sticker on the back.

Press and hold reset when turning on your ps2 for 3 seconds then release.

Sorry for this question, but I'm a total PS2 newb. Do you mean turning on the PS2 with the switch on the back while holdig reset, or pressing in Standby reset for 3 seconds?
Either way, nothing happened. The PS2 still won't read CDs or DVDs...
Only PS2 Games (which is not all that bad ;) )
What should that 3 Second reset do?

Pressing and holding reset for 3 sec from standby should enable PSX backups and dvd movies to load. Still it depends on what modchip you have.

yeah what Warlock said :D