MSD stands for missing signal detected,
and the last time that happened to us, we ended up replacing the transducer. It turned out to be a bigger project than I expected, since the old transducer was no longer being built, and the replacement needed a new thru-hull. I experimented with a cheap Hummingbird fish finder that I siliconed to the inside of the hull till I had time to haul and repair the Datamarine. I kinda liked the fish finder (I'm cheap) and liked watching the fish under the sailboat. The silicone is tricky and keeps losing it's seal and needed constant re attaching, but it was lots better than MSD...lol. Good luck!