CDR works, then next minute "Unreadable"?



I was reading a burned CDR and it was working fine. Took it out, read another disk, then put it back in and now I get an error “The disk structure is corrupted and unreadable”. Why would this happen? Disk doesn’t look scratched or anything. Tried it on another drive with same error.

Both optical drives tried are older Plextors running of a SCSI card. Media is a Verbatim Datalifeplus (obviously misnamed!). Was burned at work off an old HP burner (which is a coaster machine). Anyway to get this disk back to readable?