AUTOSCAN is disabled for devices that have the norewind field specified in the brutab file (since BRU cannot rewind the device, it won’t be able to read it). It is also disabled when writing to devices with unknown parameters (devices that are not listed in the brutab file).
AUTOSCAN is automatically disabled if the -i or -d options are specified – that is, if BRU is invoked in inspect mode or in differences mode. (See Appendix C, “The BRU Manual Page,” or Appendix D, “Table of BRU Modes and Options,” for a description of all of BRU’s modes.) Since these options also read the tape and detect any errors in the data, AUTOSCAN would be redundant. You can disable AUTOSCAN by specifying the noautoscan field as part of the brutab entry for a device. This is not recommended, however. For most tape drives, this scan of your archive tape will take only a few extra minutes (it is typically much faster than writing) and it can help you avoid many future problems.