Many AVCHD video cameras write large video in many MTS fragments. For example, there are typical flash card files from a Sony NEX camera:
00123.MTS 1905 MB 00124.MTS 2026 MB 00125.MTS 2026 MB 00126.MTS 783 MB 00127.MTS 2003 MB
Here 00125.MTS, 00126.MTS and 00127.MTS are parts of the same recording session.
To use these spans in any video editor or media player you should join them together.
The Simplest Way using Copy
Use a Windows batch command:
copy /b 00125.MTS + 00126.MTS + 00127.MTS out.mts
In some cases this way may not work, so use next methods:
Join MTS Files with no AC3 Recode using FFmpeg
ffmpeg.exe -i "concat:00125.MTS|00126.MTS|00127.MTS" -vcodec copy -acodec ac3 -sn out.mov
AC3 is not supported by DaVinci Resolve, so you can use next method:
Join MTS Files for DaVinci Resolve using FFmpeg
ffmpeg.exe -i "concat:00125.MTS|00126.MTS|00127.MTS" -vcodec copy -acodec pcm_s16be -sn out.mov
Please note that very long sequences may not be placed in MOV container. You can try MKV instead. Or make MP4 with a near-lossless AAC encoding:
Join MTS Files to MP4 using FFmpeg
ffmpeg.exe -i "concat:00125.MTS|00126.MTS|00127.MTS" -vcodec copy -acodec aac -ab 512k -cutoff 22050 -sn out.mp4
Please note that AAC encoder from FFmpeg has very poor quality, so we use 512k. You can try 320k or less if your audio record is not important. For old FFmpeg versions add -strict experimental.
Using Video Artifact
Extended editions of Video Artifact can join camera source files when importing using third party FFmpeg.exe. They select required spans automatically. But a manual joining is required if you use a Basic edition.