NA & AU [also ZA i guess] versions of 3597 don't come with a microSD inserted in the unit as the EU version does. It seems from the above info that the card fitted in the EU has been 'locked' by having a GarminDevice.xml written to the card prior to packing. The NA & AU versions do not write an xml file when a card is inserted. I have an NA 3597LMTD purchased new direct from the States. I've run it with AU maps etc on an 'after-market' Lexar 4GiB since i've received it but no xml has been written to the card by the unit. Similarly with an AU 3597 to which i've had access.
These units differ to other modern nuvis in that the internal path to the card is not 1:/ it's 2:/ as it was with early 3x0/6x0 nuvis. In 3597, 1:/ is to the unit's RWFS which contains another .System folder with sub-folders Diag, GPS and SQLite folders. It is possible to write to the RWFS using commands, e.g. an NV backup using conventional path of 1:/ will write to it.
When my 3597 is connected to PC with a card inserted, in Win8 they appear normally under 'Safely Remove Hardware and Eject Media' as this:
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and must be ejected from there together. Only under 'Computer' may they be ejected as separate devices. I expect that in your case
isg, the unit and card will show as separate devices even in 'Safely Remove etc'.