FBNeo is strict about how ROMs are stored.

Arcade boards often dump ROMs with hardware headers (e.g., 16-byte iNES headers). FBNeo expects raw binary dumps. A header-preserved ROM will produce a correct filename but wrong CRC → unknown.

Example A — checksum mismatch

Example B — parent missing

Example C — merged vs split

Example D — wrong set revision

To fix an "Unknown" ROMset, you need the DAT file.

This is the silent killer. You download a large 400MB romset via a torrent, but one .bin file inside mslug.zip is corrupt. The emulator scans the zip, sees the corruption, rejects the entire file, and labels it "unknown." Always verify your downloads with tools like clrmamepro or RomVault.

Many users rely on curated collections like "FBNeo Romset Complete (Rollback)." These sets are updated sporadically. If you have Rollback set 1.0.0, but you updated FBNeo to version 1.0.3, you will get "unknown" for any game that had a ROM update between those two releases.