My Canon EOS 600D's class 10 SD card does not write out data very quickly and this results in buffering issues which can be seen when shooting 1080p@30fps video. Why can't the card keep up and what can I do to fix the problem?
The class of a memory card often deals with the reading speed of the card, even if it should deal with the write speed. It is relatively common for a card to have much quicker read times than write times, but since the read time is fast enough, they slap a faster class label on it than it can actually write.
You will need to get a class 10 card that actually has high speed writing in addition to high read speeds. The best bet for checking this kind of thing is to read reviews of the cards and buy from reliable manufacturers that put the write speed in their spec. The spec can be important since SD card makers will often change the actual storage chips used without changing the part number of the card.
You don't mention which Transcend card you bought but the benchmarks here indicate that the write throughput performance of Transcend cards are a little under 10 Mbps (naively I assume this is true for their brand of SD cards but I could be wrong). Strictly speaking the SD Card class rating seems to imply 10 Mbps minimum for 1080p recording so that may partially explain why you are seeing that behavior with your camera.
If you can try other manufacturers, look at their benchmarks from them as well as reviews to see how it goes.
I use an app called "Black Magic Disk Speed Test" to determine the read/write speeds of my various cards and disks which has the added benefit of also giving a go/no go status on the various standards (such as 1080p@30fps). Also, most of the time you can find user submitted images on Amazon that are screenshots of this (or similar) apps and see how users are actually reporting their speeds.
If you have success some times, and then not others, one source can be spikes in the time to write.
Some solutions are to make sure the card is recently formatted, thus all ready to write to, and no way near full, so there is plenty of space to lay down large chunks of data.
If you do the above and never have success, it could be too low a class card, and/or a fake card. I've seen reports of people buying 'fake' SD cards, that are just long end cards re-stickered. To test that a tool like the others mentions would be the trick.