iMac won't recognise .MTS videos on my hard drive, how can I play them?

Try VideoLan. It is free and should handle most kinds of video formats.

The method for recognising AVCHD format is with regard to Apple Video applications, and not QuickTime. Those applications have the reader for that format. But you're only interested in viewing them so I suggested VideoLan.

Also there is a QuickTime plugin which recognises many formats, but I have never tried it with AVCHD or MTS formats. It is called Perian. Please note that it is no longer supported in the form of updates but it is still offered on the official site.

QuickTime Player

QuickTime Player supports AVCHD files. The clips need to be converted first and must be mounted via a camera or external device. See OS X Mountain Lion: About AVCHD files and QuickTime Player for the specific details.

Watch with VLC

As others have suggested, VideoLAN Client (VLC) is capable of watching your MTS files without needing conversion.

Convert with Handbrake

Handbrake is an open source tool that can transcode video, including MTS to MPEG4. As Scott Earle suggests, it provides a graphical interface for the underlying ffmpeg tool that performs the conversion.

Convert with ffmpeg

If you want to use your MTS files in iMovie or other tools, consider converting them to MPEG4 first.

The open source tool ffmpeg can convert AVCHD/MTS files to a wide range of formats.

The simplest way of installing ffmpeg onto your computer is through the homebrew project. To install brew, launch Terminal.app and issue the command:

ruby -e "$(curl -fsSL https://raw.github.com/Homebrew/homebrew/go/install)"

With brew successfully installed, you can now install ffmpeg:

brew install ffmpeg

There are numerous tutorials for converting video formats with ffmpeg. A couple are linked below.

• Convert an AVCHD / MTS file to MP4 using ffmpeg
• FFmpeg Tutorial: Convert MTS/M2TS(AVCHD) to AVI

The final command will approximate:

ffmpeg -i 00031.MTS -b:v 4000k 00031.MP4

Newer versions of OS X (I think 10.9+) now support the AVCHD container. But if you extract just the .MTS files on their own it won't know what to do with them (which is a bit of a cop-out, because obviously apps like VLC can play them!).

If you just have MTS files, you can actually re-build an AVCHD container for them using Panasonic's AVCCam Restorer utility (both Mac & Windows).

This was a huge help for me, because I backed up only the MTS files for heaps of videos, because I didn't really understand the AVCHD container. I've been able to go back and re-make AVCHD containers for them, and then easily convert them out of that container into something simpler to manage (i.e. individual clips), without (lossy) transcoding.