Is the Sparse Image disc reliable?
 

I store, on an external disc of 500 GB, an important collection of classical music which has to increase continuously. To protect this invaluable material, I make a backup of it on a second identical disc, by using the option Smart Update of Sparse Image.
Since, for the moment, it remains still much of space available on the discs, the Sparse Image enables me to use it for other things. I wonder whether this method is sure. What would arrive it, for example, if the disc of origin made defect during a backup. The data contained on the image disc would be preserved?

It's always better to copy directly to the drive, of course -- the fewer "layers" between you and your data, the better.

But a sparse image is a standard part of OSX, and is what Apple uses for FileVault: they've certainly tested it reasonably. But it'll be less fault tolerant than a direct copy, no question.

I have my answer. It's a question of tolerance to the risk.
In this case, I cannot take any risk.

Thank you infinitely, as always.