The .EFC and .EFD are files with meta-information. The first time a dataset is loaded, the reader needs to determine where in the file the different information is stored. To do that, it needs to scan the entire .cas and .dat files. The meta files contain information about where the variables and parts are located. This is done so that the next time the dataset is loaded, it will load faster because the reader already knows where all the data is located in the .cas and .dat file. It will work fine without those files, but it will be slower.