2002 Navigation System

Bought my Prius used a few years ago.
Everything looked great and still is working fine,
but at the time that I bought it, the navigation
system would read the original disc about every
2nd or 3rd time it was inserted in the unit.
Over the past few years, of course, the problem just got worse
and I finally gave up trying to use it - always getting the
"unable to read data" message.
My question: is there a way to check thru the display
if the problem is actually with the disc or the machine itself?
Is there some kind of sequence of buttons to press to obtain this info?
I've checked some other posts here and noticed some suggestions
that would seem to work on a post-04 model, but, for instance,
I have no "navigation" button - and the "map" button just shows the
"unable to read data"message.

thanks,

Rod