When the Eternity project is built using Final configuration. There are #ifdef used throughout the code that make changes.
One of those changes is to not allow direct loading of files in the data folder but use package files only.
Code:
r3dFile* r3dFile_IntOpen(const char* fname, const char* mode)
{
r3dCSHolder csHolder(g_FileSysCritSection);
bool allowDirectAccess = true;
#ifdef FINAL_BUILD
// disable all data/ files in final build
if(strnicmp(fname, "data/", 5) == 0)
allowDirectAccess = false;
#endif
p.s. These two posts should be in Help section.