Nothing I have tried gives me read/WRITE access to my SD card from my Canon camera. How can I secure-delete before selling the card on eBay? (the lock switch is on: unlocked - tried the other position too, just to make sure)
If you can't erase the card from your camera does the camera at least recognize the card? Can you take a picture and have it saved to the card? If not the card may be damaged or your camera's card reader may be damaged.
Either way, try putting the card in a card reader attached to your Mac's USB port (or if you have a later model MacBook Pro or iMac, in the computer's built in SD card reader) and see if the Mac recognizes it and mounts it on the desktop or in a Finder window. If it does, run Disk Utility and erase it there. Generally these cards are formatted MS-DOS (FAT) for use in Canon and other brand cameras and you can choose that format in Disk Tools.
If you want a more secure erase you can use Disk Utility "Security Options..." to accomplish that.
If you can accomplish this with your computer, put the card back in your camera to see if it is recognized. If it isn't there may be something going on with the card reader on your camera.
Just to be thorough, what brand, size, and speed is this SD card? Some off-brand cards can develop issues like this (directory or database problems during a shoot, don't format correctly, etc.) and it's worth noting here that there are differences between brands of these cards. I've never had a problem with dozens of Sandisk, Lexar, or Kingston brand cards of all types: compact flash and SD of all types, sizes and speeds.
If two cameras fail to erase the card, then something is almost certainly broken in the card.
You could simply put the lens cap on and take enough black pictures to fill up the card. That is equivalent to secure erase / one pass as the old data will be overwritten.
On the Mac side - your best bet would be to repartition the drive to APM or GUID so that the OS knows it can erase everything. Some filesystems the Mac doesn't "support" fully will be marked as read only.