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I suspect that you need to format the memory card first, e.g. on a computer that has a memory card reader slot of the right size and type, to make it useful to the Garmin Dezlcam device.
The 1 Gig available already may be simply the onboard memory chips in the Dezlcam device, and nothing from the memory card slot yet owing to the memory card not being formatted and usable.
Let me know what you find out, when you have tried this, please. Thanks.