I know WHS is probably easier to set up, but Microsoft did not pioneer the concept of "drive extender". Linux had something called LVM since 1998. According to Wikipedia, IBM AIX had a similar feature in 1989. MS also had similar capabilities in NT for quite some time now (disk volume sets), they just weren't shown to the home users.

Autobackup can be done by rsync, file access by Samba, printing through CUPS. Oh yeah, you can run all of the above on an $50 wifi router with custom firmware. It will also barely use any power and not make any noise.

EDIT: I am not attacking MS, and I don't hate people who don't use Linux. Just pointing out there is a way to achieve this functionality with mostly off-the-shelf software and hardware. Startup idea? Nah, more downvotes :)