Salesforce provides the user interface to check the Limit and Used of the Storage from the Application Set Up -- Administration Setup -- Storage Usage .
But the information is not accessible through API.
If the Storage used information is available through the API then we can used the information as below:
At the time of bulk data load from external API interface
program can check (if )the storage space available and make the decision
to proceed with data load
or (else) Send an email to the Data Administrator.