The ARM7 patch or, 'The save data could not be accessed...'

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The

'Unable to access save data. Please turn the power off and reinsert the DS card.'

or

'The save data could not be accessed. Please turn of power and reinsert the DS card.'

error messages you may receive on certain games is due to a newer executable system file used in these games, known as the ARM7 binary.
Older cards (such as the R4) are not compatible with this version of the ARM7 binary and will fail to run the game correctly, this is where you see the error message.

The first thing to do to get around this error is to update to the latest loader/firmware for your card. This should be obvious and easy to do, check my other guide to see how to do this if you can't figure it out.
If the game is still having problems, then you will need to replace the problem ARM7 bin with an older version, which does work - this method has been come to known as the 'ARM7 Patch' or 'ARM7 Fix'.
To perform this, you will need: 1. The problem game ROM, 2. Another ROM that does work correctly, 3. A ROM patching or extraction app.

For #3, you can download gui_arm7_102.zip. This is a small purpose built program to do the patch (thanks Bad_Ad84).

Once you've downloaded it, extract the two .exe files into a folder somewhere (this is for Windows, if you have another OS I'm sure you can figure out your own method).
Open up arm7_gui.exe. On the Tools menu, choose Extract ARM7.bin. Now, find and select the #2 ROM, the one that does work correctly on your card.
This will extract the ARM7.bin file, save this somewhere.
Now, on the main screen of the patcher, for the first box, select your #1 problem ROM. In the second box, select the ARM7.bin file you've just extracted. In the third box, choose somewhere to save the patched ROM file.
Then, click Go. If you've done everything right, the game will be patched and saved in the location you chose in the third box.

Try this patched game on your card, it should now work.
If it still doesn't, try another ARM7.bin from a different donor ROM.
If it still doesn't, then it just won't work. You'll have to wait for a new loader/firmware for you card to be released, or get a better card.