Individual, unstructured blocks with a maximum size of 1 MB, that can be evicted at a second’s notice don’t have to add to the “fullness” of a drive, if it was organized with that in mind.
One could even modify a file system so it would lend its free blocks as opportunistic storage space for SAFE. Which is a pretty decent idea, if somebody is willing to pick it up 
If we needed some space (e.g. for a new file), the FS would just do its job, but it would also compile a list of the chunk copies that got deleted in the process, and then report them to the storage manager nodes (or whatever they are called) at the first opportune moment.