HP 20/700 - Manual (Page 181)

180 9840 Drives and Tape Cartridges Appendix A
Write-Protecting 9840 Cartridges
You can set the write-protect switch so the cartridge is write-enabled. Slide the
switch to the left to write-enable the tapes. In this position, the drive can write
as well as read data. This setting is recommended when inserting cartridges
into the tape library.
You can set the write-protect switch so that the cartridge is read-only (nothing
can be written on the tape). Slide the switch to the right. In this position, the
drive can only read data from the tape, but cannot write data to it.