You're using 90 minute CD-R's? It could be that your Plextor drive doesn't recognize these discs, or the drive is detecting the wrong size. Which firmware does your Plextor drive have installed? Firmware 1.04 adds correct identification of >79 minutes media.
Another question, why are you using 90 minute media and not standard 80 minute discs?
Also, have you installed Alcohol software? It's known to cause some problems with media detection. See this thread for example.