Our Partners and Consultants have mentioned SpiderOak at CLE programs over the last several years because it is a privacy-focused Cloud storage company. With SpiderOak, you and you alone keep your encyption/unencryption key. That means no third party will have access to your data. Even if someone could get their hands on the raw bits that make up your firm and client files (very unlikely), they have no way to unencrypt and read the files.
Of course, if you lose or forget your key (aka password), there is no one to help you. Your data, while still there, is as good as gone because it cannot be read by you or anyone else. As they say, with great power comes great responsibility. It is all in your hands.
As with other Cloud file storage providers, SpiderOak just dropped its fees substantially. The small 2 GB plan remains free, but if you want to put all of your firm's data in the Cloud where it will be safe from the type of local network hacking that befell Sony, a 1 TB plan at $12 monthly or $129 if paid annual makes more sense.