write protection switch
Computing term for a physical switch on storage media that prevents data from being overwritten or erased.
If you lock the write protection switch on an SD card, you won't be able to delete data.
書き込み禁止 is a general state or setting of write prohibition, while 書き込み保護スイッチ specifically refers to the physical switch that enables write protection.
Compound of 書き込み (writing) + 保護 (protection) + スイッチ (switch). A straightforward technical term.