I found this:
Its the manufacturers manual for an MMC card. It states that the Card Identification Number ... CID, can only be written once into the card data register. This is done by the manufacturer prior to shipment to sales outlets. Each CID is unique. The CID can be read with the correct software.
I think we are definately scuppered. Either the Chart Plotter will need a software hack to allow it to decode the charts no matter if the CID did not match the CID encryted into the charts ... or we need software to decrypt and then encrypt back again using the destination CID.
I have given up this quest, too much effort and highly illegal if done. Just not worth the effort for a personnal backup.