Is the message "No Disc Detected" even when you have a known good NAV disc in the drive????
Because if so, I had the same problem in my Lexus RX300. It was a DENSO unit. I called the
Audio & Video Service Stop. They quoted me $385 for the repair. Since I work 35 minutes from their location I dropped it off. I had a twenty minute conversation with Jim, the repair tech (Tom is the other tech, owner I think). Jim's a pretty nice guy. Here is what he told me.
There are not a lot of people who repair these because DENSO does not publish any specs or repair manual on their equipment. The "Audio" guys figured out how to repair them thanks to a local dealer who was desperate to sell a used Lexus that also had a "No Disc" error. According to Jim, the dealer was going to be charged ~4+k by the local Lexus dealer. This was going to blow any profit for the re-sale. So this dealer was desperate. The "Audio" store did not yet have a test rig to check out the hardware so the dealer actually loaned them the car for as long as they needed it to troubleshoot the problem. They tracked the problem down to the motor that drives the laser. It's turning out that this is what is wrong with most of the Lexus units they are seeing. This was what was wrong with mine. I dropped it off on Friday and it was ready on Tuesday. It now works as good as new. He did say that they have no long-term experience on how long the repair will stand-up. The replacement drive motor is a different model than the original but it has a higher spec. So in theory it should last longer than the original.
Some additional info:
There are several Lexus dealers who are now sending him units to repair for the $385 price and then charging thousands to the customer. F'ing disgraceful if you ask me.
The "Audio" Store is a little shack of a building. You would never guess it based on their web site. 2 techs and 2 girls to answer the phone. They like to give the impression they are bigger than they are.
This Lexus Nav problem is a relatively recent but growing phenomenon. They are now getting 1-2 a day for repair. There was another one on the counter when I walked in to drop mine off.
Any moving part WILL eventually break. Every time you start your car, these built-in Nav units come on and read the disc (i.e. the moving parts move). So, if you want to stress your DVD-based NAV system less, only insert the DVD when you are going to use it and eject it after. Of course keeping the disc safe if it's not in the drive is another issue.
They were unable to repair a few units because the owner attempted to repair it themselves and ended up damaging it further.
Wow. For this comment, I got put on ace10's ignore list. Some people are a little sensitive. 
