If you have a camera (like my Sony RX-100) that produces .MTS files for its H.264 format, then editing on the mac ain’t easy. Because you really want something like mp3trim which trims but doesn’t uncompress and recompress.
Every once in a while I try to edit video in the .MTS format on m yMacBook with the built-in free software iMovie. Sometimes it works. Most times not. [But it converts it to a huge AIC uncompressed format to do this]
via Apple frustrations with .MTS video files – RickMcCharles.com.