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Here are some facts about 3x0. No hearsay or from another forum or even from Garmin, but from my experience with a couple of 3 series. The unit can see at least a 8GB Sandisk SDHC formatted in FAT32 [8 'GB' = 7,939,817,472 bytes = 7.39 GiB]. It can use Full NA 2014.20 which is 1.88 GiB binary (2,025,357,312 bytes, or ~2.025 GB decimal) but it cannot recognise AU Topo V4 which is 2.84 GiB (3,050,766,336 bytes).
This is definitive as far as i'm concerned: 300 series is not limited to 2GB card, but is limited to under 2GiB map image file.