I have a 2597LMT and used to have the EU maps installed on a 16 GB SD. Now Express correctly states that the EU maps will not fit in the internal memory, and suggests installing on the SD. Turing the installation, at ca. 75%, the installation ends and a red title bar with the message 'there was an error installing the maps' is displayed. This occurss repeatedly, and both with x.9.0 and x.10.o Express for Mac versions. The SD has been reformatted (Fat32) and is seen by the Mac.
Interestingly, the Windows version of Express does not seem to see the SD and instructs installing a smaller regional map. This completes w/o an error, but in addition to the missing map, some functionality (e.g. Finnish voice commands) is lost.