Canon 600D showing card write protect switch is set to lock, even though the SD card was set to unlock mode/read-write mode. Can anyone please help me to resolve this issue?
Is the phisical lock in the unlocked position and not broken? Try moving the switch multiple times to be sure that it is not oxidised.
Then try the card also on the PC or another camera.
Also verify in Windows that the folder does not have some strange permisisons applied. If oyu can only read it, then download all the files oyu have on it and then format the card with the camera or pc.
EDIT BASED ON YOUR NEW TITLE
You say "when switch is set to lock". the switch should NOT be set to lock. when it is in lock mode you cannot write!