The Carminat units have no internal memory, so not only the map data but also the operating system and firmware have to be included on the card. Only rarely will someone set up a 720 to boot off the SD card, so there's very little chance that the SD card had any OS or firmware on it to run the Carminat unit. THEN, even if the 720 firmware would have been on the card, and would have run on a Carminat unit, you've got the problem that maps are keyed to device serial numbers, so had the Carminat booted up, you still wouldn't have had maps.
Too bad the original owner didn't leave the card for you.
Not trivial. Normally, a fresh copy of firmware would be triggered by connecting the 'empty' device to a USB port with TomTom HOME running and a freshly formatted 2GB SD card inserted. However, not so easy in your case. There, it would do so if it found an SD card inserted into a card reader on your PC --- but it would have to be a card with at least an OS on it, which you do not have.
I think dhn's suggestion to contact support is your best next move.
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