Thinking back to yesterday, and without my crankypants headache face on, I think the sequence of events was something like this:
1) Turn unit on.
2) Try to reset the max speed in the aquapack, but couldn't because the thumb switch was too fiddly (and aquapack too tight)
3) Take unit out of aquapack.
4) Reset max speed
5) Put unit back in aquapack.
*** DANGER Will Robinson! ****
As I was putting it back in, the aquapack pushed the thumb switch up, and possibly in. When the unit was being inserted, all the options were flashing up on the screen.
Possibly what happened was that the unit selected "Format Card" as I was putting the NAVI into the pack.
6) Seal up and go sailing.
The moral of the story is:
1) Reset your max speed at home when you download data.
2) If something weird happens on shore, turn the unit off, then back on. It's better to have two files that you have to stitch together, than no file