[jo909]

where do you create that checksum? If its on a computer without ECC, you will just checksum the data including the error, then write that data including the error to disk.

What happens to the data after you have read it to memory and successfully verified the checksum? You probably process it in memory, and have no idea afterwards if the changes are due to your code, or because of errors.

Of course you can now propose to also checksum and check the data while it is in memory. Which is basically what ECC does, in hardware, for cheap, requiring no CPU cycles.