The switch is tiny an can be easily overlooked. Some switches are more obvious and are located on the left hand side. What the make of the card?
It might be locked to a certain computer that has been known to occur. Or corrupted.
What I would do is do a back-up and get everything off the card before doing anything else. Are you using the phones software to copy the music over to it ?
Have you tried using the card inside the phone it came with (assuming its in one) As some card readers have been known to throw this issue up and you have to update all is software. I know some laptops when they format a card, when you place it back inot the camera, for instance, the camera wants to reformat it so it can be used. When you do that the laptop then states the card is protected again. Some cards work like that other work fine. same goes for Card readers, some have a tiny problem that causes this by a tiny bit of solder that is needed to secure a loose part.
I assume if it is in aphone you actually set the phone to allow it to be taken out not just took it out? With my old nokia to remove the phone card I had to go into the menu and set it to eject the card then I could remove it. If I didn't it would throw a right good hissy fit and the read/wite error would turn up.
Personally get the card backed up, then format it and see if that works. That or attach the phone and use its software to drag the music over. Alternatively open window lob out card and shut window.