Archival data protection

Claim 1

In a storage system having a plurality of separate storage units, a data protection system for preventing the loss of more data in one of the units than is correctable by an error correction and detection scheme to protect the data in each of the units, comprising: a check unit containing check bits for a plurality of the storage units on a bit position by bit position basis wherein each of said check bits is the Exclusive OR summation of the bits of a single bit position in all the storage units in the plurality of storage units; update means including two read before write station means that read the data in a bit position of a separate one of the storage units and the check unit before writing data in the same bit position for updating check bits of the check unit each time a bit in one of the plurality of storage units is changed said update means including means in one of said read before write stations for obtaining a first Exclusive OR sum of the original and new values for any changed digit and means in said other read before write station for obtaining the Excluisve OR sum of the results of the first Exclusive OR sum and the check bit in the changed bit position to generate the updated check bit covering data in the changed bit position; restore means including the two read before write station means that read the data in a bit position of a separate one of the storage or check units before writing data in the same bit position for exclusive ORing the data in the check unit with the data in all the storage units other than said one storage unit to reproduce data in said one storage unit when the data in said one storage unit is uncorrectable by said error correction and detection scheme whereby catastrophic losses of data are prevented.