There was a thread about this in the TC forums quite awhile back.....
At one point, I had a Transcend 4GB Class4. It had a poor response time (1.6ms), but had great read/writes and worked perfectly fine in the CycloDS. But I wanted more space (and a better response time), so I bought a Transcend 8GB Class 6. Big mistake! The card was flaky as hell in my CycloDS. It would get stuck at loading, especially if it was the first time the ROM was booted and it had to create the save file. The enhanced menu would take forever to come up, or not come up at all. Ultimate Mortal Kombat would fail to load (black screens) unless I deleted the save file.
Yes, that crap actually happened. That same microSD works fine in my cell phone and a camera I tried it in. It also performed well in any tests I ran on it. If it has some kind of defect, I can't determine what it is. It's currently in my cell phone and still working fine.
Now, at the TC forums, some people had the same issues as me with the very same type of Transcend while others said that the exact same card worked fine for them. I don't know if it was a bad batch of SD's or just my CycloDS being particular, but I've NEVER had any other SD act like that in the Cyclo.....even one I knew to be defective at writing to it (RiDATA 16GB Class6).
I'd avoid Transcend in the CycloDS just to be sure you don't get burned by one of those flaky ones like I got. Best idea is to stick with the "known good" brands like Kingston or Sandisk.