Folks, I have inherited a Garmin Zumo 660 GPS unit. From information gathered on this site, the 660 will still do a RTW trip, even though it is a bit dated now ( so am I but I like to think I can still manage!) once the maps are downloaded. I know very little about the unit but am studying up using manuals and websites to fill in the gaps in my knowledge.
My question is ...... can I download all the maps of the world (road) using a micro SD card with a greater capacity than 4Gb if I need the greater memory capacity? 4Gb is the extent of the internal memory, so I probably can't (know little about computers). Opinions please. If 4Gb is the limit, do I have to discard maps I no longer need and import the ones I need for the leg of the trip I am completing? I hope I can still import maps from the new site (OSM I think it is now). Thanks in anticipation.

In theory, you could load a memory chip up with all the maps of the world, and then selectively turn on the map for the area you are in, and turn off the maps for where you are not.

In practice, though, unless you plan to complete your round the world trip in 3 months or less, it might make more sense to just load maps you will need in the next 6 months. This is because maps get updated all the time, and if you load maps now for a location you won't reach until a year or more from now, those maps will be out of date once you get there.

Don't turn on (activate) maps for regions that you are not present at. If you have too many maps active on the device at any one time, it slows the device down when certain tasks are being executed, such as start-up and route calculation.

Be aware that Garmin devices have a built-in basemap that varies depending on the model of the device. If you have an "Atlantic" device, it will have a basemap of Europe. "Americas" devices have a basemap of North America. I note that you are from Australia, it is possible that there is an Australian model that has a basemap of Oz (and probably NZ). Once you get out of the region that the basemap covers, you will lose some display capability - for example, when you zoom out to really wide views (more than 100 km wide on the screen), you might not see the elevation & topographic shading that you would normally see when you are within the home (basemap) region.

Michael