Used Garmin card with Bluechart
I have just moved S/V Jeanne Marie to the Chesapeake, but my older Garmin 178C doesn't have active charts of the area. I see I can get Chesapeake charts on the older Garmin cards for about $175, which seem steep since the charts don't go to FLA or the NE as the equivalent C-map charts do (I believe). Plus, this unit probably has only a few more years in it.
I have found a Garmin chip for sale where the seller says "I bought this unit on eBay but don't need the card, so am selling it".
Is it correct that his card (and my original card) will only work in the original unit? I asked the seller and he said "no problem", of course. Garmin's web site is useless.
Any advice will be appreciated.