Hello, it seems to me that what you need is some sort of circular buffer 72000 elements long (3600 sec * 2 * 20Hz). TDMS is not the proper instrument to achieve this, since you cannot delete elements from the file once written; I suggest you to keep all data in memory instead, and dump the whole set of data to disk only in case of failure, with a format at your choice.
A good and simple way of storing data in memory for this purpose is to have an 72000-elements array with proper initialization at program start; when you need to record a measure, you can shift all elements by one to the right and store actual measure in element 0: that way your data will be always stored in the array sorted from the most recent backwards with increasing index.
