1. Try using a PC, i've had no problems updating to microSD for multiple 3xxx devices including 2GB memory 3590 AU versions when using the Win version of GE. BTW, MapInstall only writes a detail image file from a PC Install version/gmap folder. Perhaps you meant MapUpdater.
2. Write the files directly to a microSD in a card reader with a copy of GarminDevice.xlm in a Garmin root folder. Then move files and folders as required to either the unit itself or elsewhere on the card. Voice, Vehicle etc files need to be on the unit itself as do the language-related SRD files need to be in the unit's .System/ASR folder. Other files such as map-related ASR/SRX & JCV files can function from the card's Garmin folder in appropriately named sub-folders.
3. Move everything possible off the unit if it's an 8GB unit and then move the maps to the card and replace the other files.
4. If you want to drive yourself crazy email Garmin UK for help. [Don't bother, reportedly they don't answer emails but their phone support is great].