Generally whenever this sort of thing happens... it turns out to be a bad memory card.
Do you know the exact make & model of the card. And also... this will sound like it doesn't matter but... where did you buy this card? There are loads of fake cards sold online where the card is basically junk-grade but sold in packaging meant to look legit. (I'm pretty fussy about where I buy my cards.)
We'll need to break this down due to the nature of how cameras & memory cards work combined with a mix of photos and video.
Some background...
The camera has some internal memory which serves as a "buffer". Ignoring video for the moment... if were to put your camera into "continuous" shooting mode, and just hold down the shutter button... you'd hear the camera rapidly click as it takes images very quickly. But at some point you'll hear the rate of clicks really slow down...
click, click, click, click, click, click........... click..........click...........click.
When you hear that slow down, you've maxed out the buffer. When the buffer is not full, the camera can shoot rapidly because the image data is saved in the buffer (very fast memory) and while the camera is taking more images, the camera is ALSO busy trying to save the contents of the buffer to the memory card. The memory card is "slow" compared to the speed of the buffer itself.
How many images you can take before it slows down depends on if you are shooting and saving as "RAW" format files (Canon ".CR2" files) vs. JPEG files. JPEG files are much smaller so it can store more of them in the buffer.
But the thing is... when the buffer is full, the camera wont take another photo until it has managed to free up enough space for another shot. It frees up the space by saving data to the memory card.
My first question is... have you ever lost a "photo" (ignoring videos)? Becuase the camera wont take a photo unless it has buffer space for it. If you are not absolutely certain I'd rather you answer "I'm not sure" if, in fact, you're not sure (because giving us the wrong answer can result in us telling you to buy a new memory card or to send the camera in for service and that'll result in more work or expense for you.)
Next... videos.
Videos ALSO use the buffer. The camera starts recording to the buffer while simultaneously saving the video frames to the card. But this gets tricky because while the camera has a system to avoid losing shots (it wont take a shot unless it can save it) this is NOT true of video.... the camera doesn't know if you plan to record a 10 second clip... or a 10 minute clip of video.
Canon has specs on how fast the memory card needs to be in order to keep up with the video frame rate of the camera. You can also reduce the frame-rate or reduce the video resolution if the card cannot keep up.
You basically want the fastest card that you can get that complies with the SD card spec of the camera.
Here's a website that tests various cards with cameras and posts the results of their tests:
https://www.cameramemoryspeed.com/canon-t6s-t6i/sd-card-comparison/
If the card fails to keep up with the camera and the buffer fills up.. then the camera is forced to stop recording.
Next is the question of "playback". Can you be specific... are you trying to review your video clips on the camera? Are you copying the data to a computer and trying to play them back? Where are you trying to review your clips and what software are you using?
You mentioned that you formatted the card. It is important that the card be formatted specifically as a "low level" format and specifically by the camera that will be using that card. If you move the card to another camera, reformat the card in that other camera, etc... (obviously unload your data first). Don't "share" a single memory card across multiple cameras or any other devices without reformatting the card for the next device. Do not format the card in the computer. Do not trust the factory format that came on the card.
You may simply have a bad card. Do you have another memory card?
Tim Campbell
5D III, 5D IV, 60Da
