Just attempted my first ever map update since getting the Nav V.
Express came up with message that 2015.20 was available so I loaded that first onto the Mac. No idea why I didn't tick the box for upload to device and computer, but in the back of my mind was the memory of something I read on here that it was safest to just load onto the computer first. Anyway that took about an hour and when I booted up Basecamp I could see the new map (ticked) with the old one still there too.
Then I did exactly the same again using Express but this time selected "device only" on the advanced options. About 45 mins later I got an error message to say that the device was full and the new map couldn't be loaded!

. I ejected the Nav V and found that now I had no map installed on it (apart from a base one with no detail). Reconnected and tried again and 45 nail biting minutes later everything updated fine on the device. I'm showing Europe DEM, CN Europe NTU 3D and CN Europe ALL on the device and City Navigator Europe NTU 2015.2 on Basecamp. Seems to all work fine but time will tell!
No idea why it wouldn't load onto the Nav first time. Should I have deleted the original map?
Anyway seems to have worked via my long winded route second time around. If anyone has a better way of doing this then please share.