Originally Posted by Rick_A
For the GTN, put the SD card with the new database in the card slot BEFORE you power up the GTN. When you do power it up, it will ask you if you want to update the database(s).
My GTN came with a Garmin labeled SD card. I always use that card in the GTN.
For the G3X, insert the card in the the slot AFTER power up. You will get be prompted to confirm you want to install the databases.
The G3X update can take 15 minutes or more. It depends on how many charts you update. When I do the download, I only select charts for the region I will fly in during the next 28 days - not the whole country.
Hint: If you have a dual screen set-up, download the update to 2 SD cards. That way you can update both screen at the same time.
^ this is my SOP as well. Very good advice.