2002 sc 430 navigation reads "no disk detected"

[gmorad]

So out of the blue while driving, my navigation goes out into a blank grey screen. I took it in to one Lexus dealer and they thought my disk was bad. They said I should just order the new updated one.
I wasn't all that confident that the disk was the problem so I took it to another dealer (both were lexus), and they said I my compter was bad on the nav system and it would be $3000 to replace it unless they could pull it out of another vehicle, then it would be $1800. So my problem is, I would like to try the disk obviously first to see if that will solve the problem, but I don't want to shell out $300 and have it not work, and then learn it was the computer that was broke. Anyone know where I can get the updated disk for cheaper so I can at least try it. Please Help