Hi xor
... This job can be done by adding an on-off switch by interfering with which sata data cable for this job. !? ...
That won't work. One pair of wires is used for writing to the device and another
pair of wires is used to read from the device. Writing to the device includes not
only the data you wish to write, but also commands which tell the device which
data you want to read.
... Applicable to on-off switched SD cards ...
It's not a switch. It's just a plastic tab that slides. The device that the card gets
plugged into determines the position of the tab through software. It's up to the
software to prevent data writes.
... this is possible in theory, but how can it be done in practice!?
Easy, mount the device using the "read only" parameter.