This works with the Elite Ti line and likely the HDS Gen3 which both have Wifi. This should be done preferably with an Android device but Apple and Windows devices should work fine also with an FTP program. I'm using ES File Explorer and I highly recommend using it because it has a built in FTP app that really simplifies the process of uploading map folders and deleting old ones and is considered by most to be the best Android app for this kind of use
I've always had problems with multiple maps on one SD card especially if they are from the same lake. I like to make and use several variations of essentially the same map because each has their special purpose and also for testing different maps on the water. So until now that meant multiple SD Cards which is frankly quite stupid having to do on a device running Embedded Linux on a dual core ARM Cortex A9 CPU but that's another story and here is the workaround
Move your maps in their own named folder over to a folder on your Android device. Insert an SD Card into the finder with one map on it (or none) and then go to your Android device and pair the WiFi with the finder. Open ES File Explorer. Scroll down if needed on the left and select Network and then select FTP from the list that drops down. You should see a folder with the IP addy of the finder, click on that and in the next window click on "Memory Card" and you are on the root of the SD Card. Now click "Local" on the left, choose the second "Home" on the list and that will open up a new tab in your home directory. Go to the place you put your maps and select and then hit the "Copy" button at the bottom. Now swipe the screen to the right and that takes you back to the FTP tab and the root of the SD card in the finder and then hit the "Paste" button at the bottom.. This will take a minute or so depending on map size because of the slow write speed on the finder. Now delete any maps folder you had on there previously. (Note that when you delete the file on the SD card the map is still showing, it loads the entire map into RAM and storage) Unplug the SD card, wait a couple of seconds and then reinsert and your new map should pop up. Lather, rinse, repeat as needed. You can also transfer some other files by clicking the "userdata" and then "logs" folder or upload and download wallpapers and screen shots in their respective folders. However your USR data, ie.Waypoints, Routes and Trails, are in "protected memory" and have to be accessed by the browser interface documented on my blog (Link below)
If you are using an FTP program on Windows the download port is 21 and the upload port is 20 but if you use ES File Explorer on Android you don't have to bother since those are standard ports for FTP and it's ready to go out of the box The Elite Ti and likely the HDS Gen3 (Or any other Navico device with WiFI) has both a simple web server that displays
this webpage (No not my blog page, the page on the finder in the pictures ;^) ) and has a simple FTP server.