Look at the files on the SD card you are trying to update. You should see one folder named "Garmin". Open that folder and you will see sub-folders. One folder will be named "JCV". Be careful as some folders from units like the Zumo 660 & 665 are different. One contains one JCV file while the other contains 2 files.On my 2017 it was a matter of renaming the jcv file to match the original file name. I believe there is a config file where the name of the jcv file is referenced. That has to match, either rename the file or change the reference in the config file.
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I have a 08 and have 2019 maps on my system. I couldn't put all 48 states on the card to keep it under 2gb I left out the extreme top left corner of the country living on the east coast it's no big deal. I have to look at the gmapprom.img file and see if the files are there and if so then find them in the gmapsupp.img and update them in the gmapprom.img. I wont have time till next week to even start looking at this stuff. I will keep you posted on my findings.