How does WORM microSD/SD Card work on the low level?

I've been reading about Write-Once-Read-Many (WORM) microSD and SD cards. The idea is that the card will work like an old CD-R/W disk, where you can write only new files. Overwrites or any changes to existing files are not allowed. (Reads are allowed as many times as necessary.) These cards are usually used by law enforcement agencies.

Here's an example of one and here's another one.

What I'm trying to understand, is how do these cards work on the low level? In other words, how do they implement this WORM restriction?

Golden said:

Write once read many - Wikipedia
Looks to be via physical switch, or encoded in the firmware.

Yeah, I saw that too. I was curious about specifics. Firmware, then how? Also physical switch -- how exactly would it stop overwrites?

The WORM restriction is very simple. Standard conventional and SSD drives have some means of keeping track of which blocks have been written to and which ones are free. This is often done with a bitmap with 1 bit for each block. Once a bit is set the block it represents is no longer eligible to be written to. With a WORM drive there can be even simpler methods. For a programmer with a little ingenuity, and most have a lot, this is simple.

Early WORM drives were like a standard CR-R disk. Now they are generally solid state. Such a drive has a lot of technical advantages over standard flash drives. Even with SSD drives you can't directly write to a block that contains data. It has to be erased first. In a WORM drive you can dispense with that. In fact you can use media that is no erasable. Since data blocks cannot be rewritten you can dispense with wear leveling. Many things are simpler and you can have a higher capacity drive at the same price as a conventional drive.

WORM drive is useful for archival purposes. A home user could use one for storing image and audio files. You can be assured the files will not be deleted or modified. And the only way they can be corrupted is through physical failure of the media.