A system includes a storage device, a backup storage device, and an intermediate storage device. Backup is on-line and copies data blocks from the storage device into the backup storage device, optionally through the intermediate storage device. When a write is directed to a data storage block has not yet been backed up, that data storage block is copied from the storage device to the intermediate storage device, the write is executed on the identified data storage block, and the data storage block is copied from the intermediate storage device to the backup storage device. In case of an error accessing a block on the storage device, the block is marked as invalid. The system suspends a write command to the storage device during the backup if the intermediate storage device has reached a selected capacity; and copies a selected amount of data from the intermediate storage device to the backup storage device.