If all the disks failed it could be the recording drive, assuming they weren't separately verified at the time or writing or since.
Our school's collection of BBCs included one drive that could read everything, including data it had created, but most of the other drives would reject anything it had written. Once identified that drive was taken out of general circulation and only used to read data for writing to a new disk in a second drive, and eventually skipped once it was unlikely any data someone cared about had been written by it and not copied elsewhere.
This often comes down to regularly testing old data. Floppies shouldn't have been used for archive storage but often were, with a problem only being noticed years down the line when the data needed to be read. Other media similarly: recorded CDs and DVDs from a decade or more ago are sometimes not readable now. An archive is only an archive if you can actually read it, much like an untested backup is just a hope not really a backup.
Our school's collection of BBCs included one drive that could read everything, including data it had created, but most of the other drives would reject anything it had written. Once identified that drive was taken out of general circulation and only used to read data for writing to a new disk in a second drive, and eventually skipped once it was unlikely any data someone cared about had been written by it and not copied elsewhere.
This often comes down to regularly testing old data. Floppies shouldn't have been used for archive storage but often were, with a problem only being noticed years down the line when the data needed to be read. Other media similarly: recorded CDs and DVDs from a decade or more ago are sometimes not readable now. An archive is only an archive if you can actually read it, much like an untested backup is just a hope not really a backup.