Last edited by serwistk; 11th April 2015 at 12:19.
I found your forum in my search for a way to unlock some Garmin maps I bought on an SD card. I haven't finished the process yet but I thought I'd add info about another way I found to get the serial number of an SD card, seeing as newer Garmins don't seem to allow this.
There's an Android app called "SD Insight" which will read this info from the card in your phone, you can get it from the Play store as usual. So, put the card in question into the phone, and run the app. It will show a screen with the card manufacturer's logo, the card size and manufacture date. Click the icon at the top that looks like an eye, this leads to a more detailed screen which shows lots of info about the card, one of the items is the serial number.
I hope this is useful to someone.