Did you put a microSD card in it? Certain ones have been known to cause issues with storage memory. If you bought it used, pop the back off and check if there's a microSD card. If there is you have a few options.

1. Plug into Zune and check for updates. (Some of the Focus updates have helped resolve microSD issues)

2. Remove the microSD and do a hard reset (Hold power button+volume down+camera until the reformat screen appears, follow on-screen prompts) You'll be back down to 8GB storage with this method, but at least your phone will be stable.