title: Read/write binary data on SD using Arduino
body: <p>I'm working on a project with an Arduino, and I'd like to be able to save some data persistently. I'm already using an Ethernet shield, which has a MicroSD reader.</p>
  
  <p>The data I'm saving will be incredibly small. At the moment, I'll just be saving 3 bytes at a time. What I'd really like is a way to open the SD card for writing starting at byte <code>x</code> and then write <code>y</code> bytes of data. When I want to read it back, I just read <code>y</code> bytes starting at byte <code>x</code>.</p>
  
  <p>However, all the code I've seen involves working with a filesystem, which seems like an unneeded overhead. I don't need this data to be readable on any other system, storage space isn't an issue, and there's no other data on the card to worry about. Is there a way to just write binary data directly to an SD card?</p>