In the traditional NetWare file system, a disk's directory entry table (DET) and file allocation table (FAT) contain address information that tells the operating system where data can be stored or retrieved. If the blocks containing these tables are damaged, some or all of the data might be irretrievable.
NetWare greatly reduces the possibility of losing this information by maintaining duplicate copies of the DET and FAT on separate areas of the hard disk.
If one of the blocks in the original tables is damaged, the operating system switches to the duplicate tables to get the location data it needs. The faulty sector is then listed in the disk's bad block table, and the data it contained is stored elsewhere on the disk.
Every time the server is turned on, the operating system performs a consistency check on both sets of DETs and FATs to verify that the two copies are identical. If both sets don't match, a warning is sent, and the network administrator should run VREPAIR.