Any thoughts how the write-protection on SD sand card be disabled?
From my information, the SD card's hardware does not control the write protection. Reader's driver checks if the write protection is on or off and allows/disallows writing. I remember that there were bugs in some GNU/Linux SD card reader drivers where the reader didn't check is the card is in read-only mode or not, so there is hope yet. I'll try to dig up specific version where this happens.
In the meantime, here's a similar question on SO where it is mentioned that some Canon's cameras do allow writing on read-only SD cards. Here's a link to wiki of the project which makes modified firmware for Canon's cameras which allows writing on locked SD cards.
After some more reading, I've found out that there are two ways to enable write protection: First is using the tab. Standard SD card readers will have sensor which will tell in which position the tab is. It can sometimes be fooled by adhesive tape for example.
Another way is by setting a bit in SD card. This way is a bit more problematic because firmware of standard SD card reader should detect the setting and at its level disable writing. This may be circumvented by using a reader whose firmware will not honor the settings on the card. Obtaining such reader could be difficult because such reader is either going to be of really low quality or custom made.
I also forgot the Captain Obvious answer so here it goes: Make sue that the tab on the card is not in locked position.