Program reading files on a wrong drive

Hello,
I have a program installed in an external hard drive ("H" Drive) and it would read data from this HHD. Lately, i installed Win 7, and the program doesn't work any more. It would only read files stored in my recovery drive ("D" Drive). Why? Whenever I run the program, i was asked to select a folder that has all the data. When i select the H drive, data folder, i get a message box saying "files were not find, please make sure the correct letter drive is selected." All the files are stored in the H drive data folder, why program can't read the folder. One thing i notice is that when i moved part of my data folder to D drive, the program was able to read them? why? My D drive is pretty small, it doesn't have the capacity to store all my data files. Can someone please kindly help me to solve this issue. Thanks allot for your help in advance.