I recently received an email about the latest Garmin Map Update for Europe 2012.40, handy for the trip to Austria! So I happily plugged in BMW Navigator IV and started the upgrade, this was the start of my problems, hopefully this post may help you avoid these difficulties.
Unfortunately the new update 2012.40, with more detail and coverage, will NOT fit on to a BMW Navigator IV or Zumo 660 which has 4GB of internal memory. So what to do. Well the answer after speaking to Garmin Technical Support is that you need to install the UK & Ireland ONLY to the Internal Memory. The best way to do this is via the Garmin MapUpdate programme which is downloaded from the web via your myGarmin account.
You will need a 4GB microSD card for the rest of the maps about £6 from Tesco. With this installed you then use the Garmin MapInstall program to select the remaining bits of Europe you need and these are then installed to the SD card.
Once this is all done the unit will happily use both maps. Full details are contained in this document from Garmin Technical Support. Caveat Emptor applies and if in doubt you should speak to Garmin but this might save you a few hours of frustration! The principles are the same for the older Garmin units and Mac or PC but the program names may change.