Yes, the Urban Rider does replace the Rider 1 and 2.
Funny thing about the Urban is that it does not come with a charging dock for your bike, just mearly a mount. This means you can operate the device on your bike for only a few hours before looking for a place to charge it. Not very convenient at all if you ask me.
You can buy seperatly a charging dock for your bike, but that seems very idiotic to me, and I suspect sales of the Urban will be so poor that TomTom will be forced to either include a charging dock or drop the device completely.
I dont believe the Urban series is even available yet in North America. The "old" Rider2 uses a 2GB SD card which nicely holds the NA map, and many users have been successful using up to 4GB SD cards (if you can find them anymore).
As far as backing up the unit, you should always make backups on your computer using Explorer, you could then put them on a card for keeping them in the event your puter crashes, etc. Do not rely on the TT HOME program for saving the contents of your device.
I was unable to find any information on how big the storage drive is on the Urban, but I note that the Europe map can be had on a 4GB SD card, so I assume the Urban has a capacity of somewhere in that range.
I have had the Rider2 for several years now, and have had no real issues with it, and it has been on my VStrom in all kinds of Canadian weather and road conditions.
Cheers