Do you have the camera set to record AVCHD?
If so, the Mac hides some info that might help you navigate to your files. A folder named AVCHD receives a special icon from the OS, and, it’s not openable the way other folders are. Instead, you right-click on it and select reveal contents. Then, open a folder that might be called DCIM, and look for another “special” folder called BMDV (I think), and right-click/reveal that. (If you don’t find the BMDV folder in DCIM, look around). Then, look for a folder called Stream. That’s where the clips are. I’m not familiar with the 5DM3, they might be .mts files?
If this is what you find, just copy those .mts or mp4/whatever files.
Or, from Premiere (other software?) you can use the media browser, navigate to the AVCHD folder, and all the clips should be revealed.
In the future, you could continue to record AVCHD, or, try MP4, which fills the card with a more conventional set of directories and file.
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Photos, Video & Pano r us.
College and workshop instructor in video and audio.
70D, Sigma 8mm, Tokina f2.8 11-16, Canon EF-S f2.8 17-55, Sigma f2.8 50-150 EX OS, Tamron 150-600VC. Gigapan Epic Pro, Nodal Ninja 5 & R10.