Garmin have been up to there old tricks and effectively trying to make older models useless despite saying that the units are LM or lifetime mapping. They have effectively increased the map details to such a point that when you try and load it, the 660 says you have insufficient memory. It does allow you to download areas of Europe, but these are only small, so say you pick France , you cant get Germany or any other country etc etc. not much use if you are taking a trip across Europe. We are not experts so if anyone has a better fix then please let us know, but this is what Michel L'Hours has come up with. Note: I also have a 590 and the maps went straight in no problem.

Effectively you download the new maps onto Basecamp on your home computer then use map install to choose the areas you want and transfer the information into the Garmin. Bit fiddly but it seems to work.

Updating your mapping Garmin 660

1. Start your unit, then select Settings, Maps, and Map Data to show current loading, untick previous maps i.e. 2021.1
   
   
2. Connect your 660 to your computer, once the computer recognises your unit Garmin Express will open and tell you that updates are available. Do not bother trying to upload the new maps to your unit because it will tell you that you have insufficient memory for the whole of Europe. It will allow you to upload certain areas but these are so small if travelling across Europe from one country to another they are effectively useless. Instead

3. Select Installation Options (important) then Download to computer only, this takes a while depending on your line speed.
   
   
4. Once the upload as finished open Basecamp and check under Maps in the toolbar that the correct update has happened i.e. 2021.2   

5. With your unit connected, on the left of Basecamp you will see your Garmin unit listed along with Internal Storage and Memory card.
   
   
6. if you right click on either internal memory or Memory Card (user data) you will see the option to Install  maps on Zumo 660 or Memory Card depending on your selection. 

7. This will open Mapinstall then press continue and you will see a screen similar to the
   old mapsource style on the 2610, showing a small map of Europe, ensure the maps are the current edition i.e. 2021.2 

8. You may now select the map areas you wish to download – if you left click it will select the area and it will become highlighted if you right click it will unselect it.
   You can select multiple areas by holding down the shift button and dragging.
   You must be very careful to ensure all the regions you need are selected as the screen is small, cannot be expanded and is a bit cumbersome.   

 You should have enough memory but the data bar at the base of the screen will indicate how much memory you are using, if it is getting too much, unselect large cities that you are not visiting as they take the most memory, and of course there is no need to select areas in Norway if you are going to Spain!

        9. Follow the instructions to complete the process.

Using the instructions in 1. above check the download has been successful you should see the new mapset ensure the correct set is ticked if more than one are there.

I have done this on two units and one send maps could not be verified and the other said maps could not be unlocked - but apparently there is a fix to this, i just have not tried it yet.

https://support.garmin.com/en-GB/?faq=3QHdSDE3Q80Vd3EHFN11u7