DISC I/O ERROR
Often happens right before a loading screen. This error appears when a file requested for loading isn't found in the CD table of contents (should never happen on an original CD, except if the TOC zone is physically damaged).
If the CD is an original one, it's certainly dirty or scratched.
If the CD is a copy, it's certainly re-authored (not an exact copy of an original). Even if the files are the same, the CD image structure can be altered and detected.
This is the case for games using file versions in their ISO 9660 structure, like Metal_Slug_2_-_Super_Vehicle-001/II. Most Windows ISO image editors strip the versioning info which is needed by the game to identify the files. Use mkisofs WITHOUT THE -N FLAGor edit the loading tables in the main PRG file.