If you download a "Non-Merged" set, BIOS files are usually included as separate zips. Ensure these remain in your \mame\roms folder:
This is the unavoidable caveat.
Create a "Working" folder by filtering your complete set using a frontend like or RetroArch . Filter by: mame rom complete set
This is the default format for most standard MAME installations. Here, the parent ROM is kept separate from the clone ROMs. The clone ZIP files typically only contain the few files that differ from the parent. If you download a "Non-Merged" set, BIOS files