On Garmins with NT mapping there are two files used for your mapping data. These have file names of gmapprom.img and gmapsupp.img. The former is the primary mapping and should contain your primary map data (i.e. City Navigator NT 2009). The latter is for supplementary mapping data such as TOPO or other regions (i.e. parts of US City Nav that you've purchased separately for a trip).
The way that these two files are updated are different with the primary mapping having a level of protection to prevent you overwriting it accidentally. This file can only be officially updated by performing a map update using an update disc, or more recently, the on-line updating service. There is a manual command line method that I wont go into here and also a method that users with a GPS that has mass storage mode can use (the 2820 has this). Suffice to say you don't really want to mess with the primary mapping space unless you really need to.
The supplementary mapping file is updated using the the send to device command in Mapsource (when you have the 'maps' checkbox ticked as you upload). It's possible that a previous owner of your GPS has used this at some time and there's data on there that you aren't aware of. Sometimes users update their mapping by using the Mapsource send to device method and wonder why everything wont fit on so it's possible that you have both NT2009 and the previous mapping on there.
When using send to device in Mapsource the existing gmapsupp.img is overwritten with a new one. This is goes part way to explain why some users get in a muddle when updating, especially if they previously owned a non NT Garmin such as a 2610 or Quest where this was the only way to update your mapping data.
Hook the 2820 up to your PC and select mass storage mode. Then check the size of both of the above files.
ISTR that the 2820 has about 2GB of memory, City Nav 2011 uses about 1.85GB, I'm not so sure about the NT 2009 version but that should be significantly less as it doesn't have data for 3D buildings, turn assist etc. ISTR that City Nav 2008 NT was around 1.4GB so it should be a little more than that.
The gmapsupp.img supplementary mapping file should be near to nothing in size or not even there at all unless you have data loaded. If it's large then it's possible that's where your NT 2009 mapping is so don't rush to delete or modify it.
If the gmapsupp.img is small and the gmapprom.img is taking up most of the memory then you can reduce this by creating a custom gmapprom.img file including only the countries you actually need. If this is the case I can provide instructions for you. However, if you don't have the NT 2009 City Nav disk and unlock code then you'd loose that mapping, you'd need to update your mapping to 2011.
Hmmm... that might negate you attempting any of the above
