First a little background as to how these old Garmin devices which don't have a visible file system store maps. They use 'regions', dedicated spaces on the flash memory which are kinda like the partitions used by a computer's OS. The basemap is in region 3. The detail maps are in (a) region 49 for the primary [OEM] map, which is currently your US 2008; (b) region 10 for a supplementary [other area] map; and (c) [maybe, but usually not populated] region 50 for another very tiny map such as a custom POI map, e.g. for dealer locations. The main 2 obviously are in 49 and 10 and in your 2720 those 2 regions share space. What that means is that if there's a large OEM primary map in rgn49 then space is reduced accordingly for a supp. map in rgn10 and vice versa. Both MapInstall and MapSource write maps to rgn10 where your sparkly new OSM MX map now resides and 10 was probably empty before. Can you see the new problem yet? ... It's this, if you now write new US data with either MI or MS it'll overwrite the existing MX map there.

There's 2 safe options now:
(i) Use a program like SendMap20 to delete the current detail maps to give maximum space and start again downloading the entire data you need from OSM, i.e. East Coast US and MX in one single *exe to install to PC then send as one single map to rgn10 via MI or MS.
Or,
(ii) Leave the MX map in rgn10 and write a new East Coast US map to rgn49, keeping in mind that we can't do that with MI or MS because they write to rgn10 only, so it must be done by another method such as using what's originally detailed in this thread by sending a compiled *.img file to rgn49 via WebUpdater using command prompt or bat file.... i.e. to overwrite your old Garmin 2008 map in rgn49 with the latest EAST Coast OSM you'll need to download the data already compiled as an img compressed in a zip file, extract it, change the name from gmapsupp.img to gmapprom.img then follow the other information in Post #3 here. To get the ZIP containing the IMG, choose the following download option after you've selected the required tiles:
[Only registered and activated users can see links. ]

Well, actually there's a 3rd option but it's not a particularly safe one, certainly i can't recommend it. That's to just use your currently loaded Garmin 2008 map for your trip down the east coast and hope for the best. Provided you keep a close eye on road signage and never trust the GPS over the signs you'll probably be fine I guess.