Used disk mgr to shrink partition from 700 GB. 2nd partition does not show in disk mgr. It should be > 250GB.As long as you are not trying to format the partition windows is installed on or has boot files on then you can do this from disk management.
If you are trying to securely erase a partition then you can do this from a command prompt using the in built diskpart function.
If you would like specific info please post a screenshot of your current disk management setup with info of what partition you would like to format
Start>diskmgmt.msc>enter