Is your SD card write-protected? On the side of your SD card there is very tiny plastic tab. If the tab is slid forward (in the direction of the electronic contacts) then that write protection is OFF. If the card is slide away then the card is write-protected.
That plastic write-protect slider isn't actually connected to anything in the card (it's not an electronic switch). When you slide the card into an SD slot, there's a tiny micro-switch (spring loaded) in the side of the slot. It is actually the card slot which detects if that plastic tab is pressing the switch in (or not pressing it in as the case may be). That's how the slot reader (and in your case, the camera) decides if the card is write protected. If the switch is "stuck" pressed in then the camera will always believe the card is write-protected.
If it is simply a dirty microswitch, then it may be possible to clean it... but there is risk in doing this because you don't want to further damage the camera.
I need to change gears at this point. Normally when I (or any of the others who frequently offer advice or guidance) supply information to others, the information I provide would not have any real risk... if I tell you the wrong exposure information to take a shot, you may get a lousy shot, but you wont break your camera. But beyond this point... that's no longer true. What I'm about to offer MAY BREAK YOUR CAMERA (as they used to label on the ancient nautical maps "Here be dragons!")
YOU need to carefully evaluate your options at this point... (a) can you destroy your camera doing this (yes), (b) are you ok with the possibility that you might destroy your camera? (c) can you REALLY not afford to have it professionally repaird? (d) do you have any experience cleaning electronics?
The caution and disclaimer here is that you are on the Internet asking for advice. The BEST advice is to send it to Canon.
While that's the best advice, you've already made it clear that this isn't an option for you. Barring any possibility of you doing that (have you actually called to inquire about this?)... you alone accept the risk of whatever may happen to your camera by trying anything else and my advice may only be worth what you paid for it... nothing. Understand that I am not a professional camera repair tech... nor have I ever been.
The next best opiton is to find a local repair shop. If you can find a local "real" camera store (not a big-box store that happens to sell cameras) then either THEY may offer repairs or they may be able to refer you to a local repair shop.
My personal suspicion is that the microswitch is dirty. But keep in mind that *I* have not looked at your camera -- so this is just a guess on my part which could be wrong.
Again... I re-emphasize that Canon service is your BEST option... and a local repair shop with some professional experience is your NEXT BEST option... and doing this yourself may be your WORST option.
You *might* be able to use compressed air to blow out the slot (and I've read reports of this actually working). You might need to clean it with a solvent. I'm exceptionally cautious about using cans of compressed air because the propellant in the can can leave a residue. The can needs to be held upright and level at all times. Also... BEFORE blowing into the camera slot, DO NOT SHAKE THE CAN .... AND...DO blow a puff of air in a safe direction (not at the camera) so that any propellant in the nozzle will be cleared out so that it wont be blown into the camera.
In fact... you should educate yourself by doing a simple experient. Get a clear piece of glass (use a hand held mirror if you have one). Shake the can vigorously, hold it so that it is NOT level (in other words, do everything WRONG on purpose)... and then spray it with the nozzle pointed at the mirror. Notice all that residue on the mirror? You're thinking maybe that's just some fog or some condesnation that will evaporate and clear. Nope... that's residue from the propellant. You will need a cleaning towel to wipe it off the mirror. It will NEVER evaporate or clear up on it's own. That's exactly what you DO NOT WANT inside your camera. While having this residue on a mirror is easily wiped away... you cannot so easily wipe away the residue if it's deep inside the electronics or optics of your camera.
Ok... with caution flags out... and with an education on why using a can of compressed air requires some care to use (it's not as mindless as it might seem), you can *try* to blow out the slot. Specifically it's probably the bottom part of the slot where the switch is located.
If this doesn't work, you could try wet-cleaing it with alcohol. It may be possible to clean it with a few drops (and I re-emphasize the term "few" drops... as in one or two... and maybe three at most -- do NOT squirt a stream of alcohol into your camera) of de-natured alcohol. DO NOT USE OIL of any kind (that would be bad -- while oil may seem a likely choice... it will ultimately attract dust and dirt and make things worse and as the oil ages it will gum up).
Alcohol has the advantage of being a solvent which evaporates... leaving the inside of the camera dry after cleaning. Remove the battery before attempting.
Note which edge of your card has the write-protect switch and note which way the card slides into the camera. That will indicate where you need to place the drop of alcohol. Place just a drop or two on the microswitch and "exercise" the switch to work the alcohol in (e.g. insert and remove the card a handful of times -- being gentle as you do.) This may disolve anything blocking the switch from functioning. After exercising the switch to work in the alcohol, allow time for alcohol to work and evaporate before re-inserting the battery. The "power" switch on the camera is really a "soft" switch... not a mechanical switch which actually deprives the camera of power. If the battery is inserted in your camera then technically the camera is "powered up" -- even if it is only in a sleeping state. Hence the need to remove the battery until it's dry so that you don't short anything out (you have been warned.)
Tim Campbell
5D III, 5D IV, 60Da
