Garmin Express 3.2.30.0/ Mac OS 10.10.2 (Yosemite) Garmin Nuvi 3590LMT device software 8.20

I manually moved NTU 2015.30 from my Nuvi 3590 to it's SD Card (32GB) to free up space so that I could have some custom POI Apps on the internal memory (they need to be there to show up in BaseCamp).

The transferred maps worked fine but to ensure that they were properly installed and would be updated successfully from the Garmin website I decided to delete the files and get Garmin Express to re-instal to the SD card.

Everytime I tried Garmin Express would just write the files to the internal memory until it was full and then fail. So I copied some redundant files to the Garmin internal memory to make sure there was only 100MB of free space.

This had the (wanted) effect of Garmin Express now recognising there wasn't enough space and saying it would write the maps to the SD Card. However it fails on multiple attempts. On looking closely at the discs (internal and SD) as the install runs i can see although Garmin Express says it is installing to the SD card it is actually writing gmapprom.gma to the .System folder on the Nuvi internal and of course the install fails as soon as the internal fills up.

Anyone have any ideas as to how to get the Express to actually write the files to the SD like it say it's doing!

I have tried Map Install and this will install the map to the SD card but it's installing 2015.3 not 2015.30 (what's this is it any different) and there don't appear to be the 3D and possible the junction views installed when i use Map Install.

Going right back to the Garmin website to source the map just asks me to install Garmin Express to update the maps... so help!!

Thanks.