Many people believe formatting the drive or deleting the file will complete delete information from the drive. Actually that is not so. A real deleting of files on drives is only possible through a complete overwrite of the original files with (meaningless) data. A simple delete as executed by all OSes makes files only non accessible on the OS and application program level (the file names are gone). But the fact is, that the complete contents of all that files are still there, waiting to be completely recovered, at least as long as they are not overwritten by new files. Formatting in most cases will rebuild file allocation data but all information still present on the drive. It means that your confidential information can be easily recovered with data recovery software.
There is only one way to prevent problems with the data you thought was deleted. You need to use wiping solution. Your private photos, passwords, financial records and other confidential documents aren't safe!