I've updated a Garmin Nuvi 760 using the Garmin Express software to the latest software version 5.40. I've since tried to update the stored maps Europe NT 2009 map using an external SDCard with various 2013-2015 NT maps copying the image over and following the folder structure. (various tutorials on youtube)

The Garmin is not seeing any maps on the SDCard. Is there a chance the software update is preventing them from showing up?

No you should be just fine. I do it on my 755/765 all the time.

760 updates will work fine if you have named the map file correctly and put them in the correct folder on SD, AND the card is formatted a FAT or FAT32.

Are you getting any messages from the GPS at boot time (ie unable to unlock map) ? If not, you probably have a simple name or location error so your 760 is simply not seeing the maps.

You SD should be set up like this.

SD:\garmin\gmapsupp.img

Make sure your map is not named "gmapsupp.img.img", a common error