I'd like to write a script or discover a command line that will do the following:

On an SD card,
Search only the first byte of each 512 byte sector for hex 0x0A.
If 0x0A exists, overwrite with 0x00.
Stop after 10,000 sectors if none found.

I've scoured the dd command and don't see a how to use it in this way or know if it's even the right tool.

This is intended to be a super fast formatting tool for a custom data recording structure. Clearing this signature byte is all I need and in theory should be hundreds of times faster than overwriting the whole card.

Any example code or direction would be appreciated. Thanks.

It won't be, in fact it will almost certainly be MUCH MUCH slower. Drives and flash chips are not accessed a single byte at a time. They are accessed in pages that are on the order of many kB.

First look at the read. Reading a full page is a single instruction, "give me page X". The card then streams it to you as fast as it can, on the order of tens of MB/s. Reading one out of every 512 bytes in a page is many instructions. "give me byte Y in page X", wait for response, "give me byte Z in page X", wait for response, etc. At that point it's not the data streaming rate that will kill you, it's the back and forth command time. I would honestly be surprised if you could send 10-100 individual "give me byte Y in page X" commands and get the resulting data any faster than you could just get the entire page in one command and one data burst.

Now look at the writes. Writing to flash is a complicated process, because you can't just overwrite a byte in flash by itself. In order to write a byte, you have to clear it first, but you can't clear a single byte. You can't even clear a single page. You have to clear an entire block (many tens of pages). That means that in order to write a single byte, you have to first read the entire block, erase the entire block, then re-write the entire block with that single byte replaced with the updated value. This is called "write amplification":
https://en.wikipedia.org/wiki/Write_amplification

You can also look at benchmarks on flash drives as a function of block size, like this:
http://www.cdrlabs.com/images/storie...pro%20atto.png

That y-axis is block size in kB. Reading/Writing blocks of 512 bytes is around 175x slower than reading/writing blocks of 64 kB.

Long story short, you're far better off just erasing the entire card than trying to work a byte at a time 10,000 times.

2 members found this post helpful.

Thank you for that insight. Though disappointing, I appreciate knowing better.

or - you read a block, modify every 512 bytes and write back in one. That will be much faster. But still slower than a simple erase.

Is this custom format something that you are defining yourself? In that case, you can more intelligently design the custom format to take advantage of how hard drives/SSDs work.

In particular, some sort of central block allocation table can be efficient, as opposed to sprinkling it all over the drive. With an SSD, of course, there are no seek delays. But even with a hard drive, this sort of thing tends to be more efficient because the table is cached in memory.

Where things get sticky is failure modes. If the power cuts out, what sort of state is the file system left in? Is it left in a weird corrupted state that is difficult/impossible to recover from? This stuff gets horrifically complex, which is why most people don't bother with creating a custom file recording system. Using an off-the-shelf file system greatly reduces headaches and development effort, as well as maintenance/troubleshooting effort later on.

These SD cards are being written RAW by a 40MHz IoT device that has too little embedded memory for a legit file system. We store 512 bytes before writing a whole sector. The first byte is simply the place we chose for a signature so it can seek the next blank sector in the event of a power cycle.