Hi Harry,
I would trouble shoot the problem the following way.
First use a different SD-card and see if you get good log files. Remember that you have to format them in FAT and not FAT 32. They are not interchangeable with the 2.4 system, as that uses FAT 32 format.
If the new card has no problems, then there might be a corrupted SD-card. I had a customer whose SD-card simply died on him. Same happened to me as well. I can't even read it from my computer. It makes my computer freeze. I replaced the SD card and everything was fine. It is important to have the receiver turned off and batteries disconnected when inserting or extracting the SD-card. (see instructions from the download section of our websites)
I am not sure weather there is a limit to the number of Log Files as well. I heard something of 250.

Hope this helps,

Gerhard