Fixing Kenwood GPS issues

I have a Kenwood DNX571HD head unit--it works fine except for the GPS.

When the GPS used, the following error occurs after just a few minutes:

"Low Mem GIR"

This is apparently pretty common and appears to have been caused by a Kenwood firmware update.

Unfortunately, Kenwood wants nearly $300 to fix it, and they need the stereo for a couple weeks.

The fix? The problem is usually caused by a corrupted internal SD Card--so they'd likely just need to replace that. This card is what contains the Garmin mapping software and data. They say that this is a propriety thing at can't be obtained aftermarket.

Any ideas for how to fix without sending to Kenwood? They refuse to just let me by the SD Card and install it myself.