Offsite backup partnering

If you're worried about losing your computer files, you've probably thought about off-site backup at one point. It's very expensive.

I have a proposal for another system. First, set apart some backup space, preferrably another computer, equal to the amount you want backed up.
Then, use a specialized P2P app to find a partner who will trade space with you, for the same amount of space that you have.
After they have accepted your backup set, your computer performs some random data checks to ensure that they are actually storing the data. The data you send them is encrypted with a password, so they can't get at your stuff.
Then, your computer accepts their data. Your system monitors the other person's backup regularly, and notifies them of an outage. At that point, you have 3 days (or some other time frame) to get ready for their backup before they start looking for another partner.
Also, your system randomly checks your offsite stuff for an outage. If it detects an outage, it will look for a new partner after some period of time.

The random checks of your own stuff are supposed to happen less often than that person checks your data. So, they should let you know of an outage quicker than you would find out. In that case, they would have a longer time period to get ready to back your stuff up again.

What is the other persons motivation to keep your stuff backed up? The fact that you are supposed to keep theirs backed up. When you have two good partners working, it works well. If you have a hostile partner, ( i.e. one that accepts your data, then dumps it), they be rooted out by periodic checks. You keep looking until you find a good partner.