At the end of today's logging trip I plugged the SD card from my GT-31 into the computer and, horror! the NMEA file was 0 bytes long.

At first, I gave up hope, and just used the few waypoints that I had successfully recorded to guess at a few path locations. At least I had something to show in OSM for all my effort.

Later, I returned to the SD card. I tried running PhotoRec on it, but the program found nothing relevant. Next, I wondered whether the GT-31's onboard data logger had saved the data, so I started NaviLink II and tried to read the log file. All I got was an error message (if you're listening, Locosys, I think we have a bug), but at least it told me that the logger's memory had some data in it. Then I decided to try writing the log file to the SD card, so I put the card back in the GT-31 and did a "Save to Card" from the Data Logger menu. This appeared to work, and I ended up with an SBP file on the card. I was then able to use NaviLink to convert this to GPX and upload it to OSM. Success!

So, thanks to the GT-31's belt-and-braces design, with logging to on-board memory at the same time as to the SD card, I didn't lose my data. It's great when a gadget gives you a second chance like this - it really feels like a dependable piece of kit now.

Epilogue: I don't know what caused the zero-length file. I admit that it's possible that I forgot to switch off the GT-31 before removing the card, but I'm not usually that careless.