I own a LG DR197 DVD Player / HDD Recorder. While playing a DVD movie it stopped suddenly. Finally it was ejected, but after that, no other DVD could be read with the indication "unknown disk".

SOURCE: LG RH289H DVD Recorder Unknown Disc. Please check disc again.

I have had the same problem a few times. All I do is eject and re-insert the disk until it recognises it, this seemed to work for me

SOURCE: LG RH265 - Problem when trying to register blank disk/dubbing

The DVD needs to initialise the disk first. Come out of the HDD dubbing menu and press DVD on the remote. The blank recordable disk should be recognised and you should see Initialise disk? on the screen. Press Start and hey presto, you can now dub from the HDD Movie dubbing menu (as per the manual instructions).

SOURCE: LG HDD DVD recorder RH7521W is not able to read

Mine had a similar problem. I assumed there might be some dust on the laser. I used an air compressor and pointed it at the DVD slot as it was opening and closing to blow some air around inside the unit (and hopefully dislodge any dust). It worked. The discs that would not play before did play after. I didn't have the air compressor nozzle too close to the opening as I didn't want to dislodge anything I wasn't supposed to. If it was dust on the laser, you might be able to clean it in other ways, such as removing the cover and using a cotton tip (take power off first!). That would have been my approach if I didn't have an air compressor.

SOURCE: LG RH278 HDD/DVD Recorder

I have an LG RH278 HDD/DVD Recorder, which was suffering from the problem that it could not play any bought DVD movies, which were previously working fine in that same player. It would however play svcd and DVD+R/-R movie discs.

The solution was to replace the laser tray. I found a new LG DVD BD lens LPC-815 laser tray here: http://www.laser-xh.com/en/?act=showproduct&proid=1147
I bought two for $5.50 each. Shipping was $32 (DHL), all together $45.10 (PayPal fees included). Delivery was fast and accurate within 3 working days (China - Sweden).

SOURCE: LG DVD Recorder "Unknown disk error"

I have the same problem with my LG DVD player, with some discs saying disc error all of a sudden. I solved the problem by cleaning the lens of the laser very gently. The player must be opened to reach the lens. It is very important to get the plug off the mains so as no electricity supply is in the DVD player. The lens was cleaned by using a soft dry cotton bud (those used to clean the ears). NO solutions liquids etc. were used. Well, till now the problem has been solved. The lens was full of dust which come from the static of DVD's. The rest of the inside of the DVD was generally clean.