The situation you describe is more akin to "format shifting" or maybe "space shifting", which is converting copyrighted material to other formats or places as a backup, for preservation purposes, or just for convenience. It is legally protected in the US, and most of the rest of the world. I believe that was settled due to early litigation with VCR's, but there are mountains of relevant cases at this point.
I do recall reading that the UK recently changed their laws in that regard, and like most copyright law it can depend on the specifics of the situation and even (*yuck*) intent. So it's always worth checking if you're unsure.
