Something from the “if you want to earn money, here is the idea”-section:
Since I tried to find the update solution which fits best to me I was thinking about different ways how to approach to this aim.
At the moment I’m using a rsync script which accesses a directory which does only exists when my external hard disk is mounted – that is nice, it spends a lot of time, works quite well, is easy to recover – and completly useless if you have big files in your directory which often change (as virtual machines or mbox using e-mail programs). But fortunately I save my vms somewhere else, and use kmail with the maildir option. Lucky me 😀
But what every backup solution misses is instant backup – when you really need a backup, the last backup is exactly x+1 days/hours/whatevers away, where x is the time ago when the information loss has occured.
So from my point of view the only, real backup system would be something which would monitor your files all the time, and update the remote backup system whenever it makes sense (for example when a file has been altered).
Today such a function is quite easy to implement (at least with linux) with inotify. Imagine a daemon similar to actual search daemons which takes information about altered files and update the remote backup database/directory about these changes.
If you use KIO-slaves, than you can define a backup path which can be everywhere (directory, ftp server, ssh server, whatever).
Ok, there is still the problem of big files, but if the deamon would be able to work with plugins you could for example provide a plugin which shows how to deal with mbox files in an intelligent way (that means: just update the new files). Or it could divide the files into different packs and use hash algorithms to check if only parts has been altered, and tries to just upload these.
Now put this together with some online web space (for linux power users the option to mount encrypted hard disks with Luks-cryptfs) and a database which can be accessed by the daemon, and you have a business solution to provide backup solutions for almost everyone. I think I really should go out and search for joint venture money – but for that I should place the phrase “Web 2.0” somewhere here :-p