My .fit file is missing a second activity — chained FIT files
If your triathlon or brick session shows only the swim, the file probably holds all of it — the reader stopped at the first finish line.
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a .fit file from a watch or bike computer
No account. No upload. The file is read on this device and never leaves it.
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One file, several activities
A .fit file is a stream, and that stream can contain more than one activity back to back: a multisport event writes swim, bike and run into a single file, and a watch that paused and resumed can chain two sessions the same way. Each has its own internal structure, and a reader that treats the first end-of-activity marker as the end of the file returns only the opening leg — losing the rest without any error.
Why it looks like data loss and is not
The missing legs are physically in the bytes you have; nothing was dropped by the device. The failure is on the reading side, and it is a known FIT trap precisely because the format allows chaining and many tools assume a file is one activity. So “my second sport is gone” is, like the partial-ride case, a parser problem with a fully-recoverable file.
Read every leg here
This page reads the whole stream on your device and surfaces each activity it contains, so a multisport file shows all its legs rather than just the first. It runs locally and uploads nothing.
If this was not your problem, the four things that actually break a .fit file covers the rest, and the file reader opens any file whatever is wrong with it.