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File recovery freeware found the activity files, but they were corrupt and I wasn't going to find any data to extract from them.
Performed a factory reset, hopefully it'll be capable of saving rides again. I suppose the safest way to get any data backed up will be to Bluetooth it to Strava on my phone as soon as it's saved, unfortunately, given that this is a Garmin, that only seems to work about 25% of the time.
Some, not much.
Usually when Garmin fucks out it's gone.