How much of that library the OP should be able to load onto his Start 20 depends upon which Start 20 (aka Via 1400 in our market) the OP has. All support uSD, but internal memory varies wildly with model. In any case,
"AO" units have 2GB
"AP" units have 4GB
"AY" units have 8GB, which should still hold the smaller memory model full Europe map
Also don't know if the firmware and web is smart enough to recognize and deal with a 16GB uSD card as 'available' for maps larger than internal memory. It used to do it properly, but I haven't pushed the issue in a while on a Nav3 device.
Can you describe what you mean by " cannot access maps"? Do you have the MyDrive app loaded on your PC, and is your device being recognized?
Please provide the first two characters of your unit's serial number. That will help us to identify your capacity for map storage. Understand that if you want full Europe, you'll need at least 8GB to do the job. You may have to settle for smaller maps, changing them out as needed if you start wandering too far afield.