if you were to store files on them their location would be something like this D:\pictures\nov1\randompic.jpeg
while if you had them stored on the C drive it would look like this
C:\pictures\nov1\randompic.jpeg
i THINK that the reason you cannot install programs on the D drive is because the OS (operating system) is stored on the C: drive therefore the registry is stored on the C: drive, its like installing a program on a flash drive that is not U3 equipped i DONT think it will work,
but you should be able to store files on the D drive
also does it say an error after you change the install location the the D drive, or does it prompt you why id had to change back to the C drive