My complete set of UK 1:25k and 1:50k MemoryMaps are on the hard drives of two computers and backed up onto an external HD as well. Whenever I've had to reinstall the operating system on either computer, I've simply downloaded the free MM software and added my maps to it.
My original Anquet maps, however, are on a CD that is too old to get a new code if it needs reinstalling, which is a bit annoying considering I once paid for it. I was about to give up when a computer-whiz on this forum contacted me and very generously gave me access to a code generator he had once made especially for it; this still worked and it saved me a lot of anguish (and the expense of purchasing replacements).
I'm a great fan of cloud storage, primarily for syncing data between computers, but I won't consider any service that doesn't have a downloadable local client, as it is called, so the stuff is in a folder on your computer as well as in the cloud. That way, if the service goes t*ts up you don't lose anything.