IBM F50 RS/6000 7025 - Manual (Page 60)

Setting the Write-Protect Tab on 4-mm Tape Cartridges
The window on the tape cartridge controls write-protection. When the write-protect
tab of a tape cartridge is set (window open), information can be read from the tape,
but cannot be written to it. When the write-protect tab of a tape cartridge is not set
(window closed), information can be both written to and read from the tape. Trying to
write to a 4-mm data cartridge with the window open causes an error.
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