Defragmenting your hard drive can enhance performance. Close all open windows and return to the desk top. A simplified explanation of what scandisk does by comparing it to an index in a book. Scandisk checks your hard drive for errors. Every disk drive has a directory and it's much like an index in a book where a subject is listed with the page number to find it.
Directory entries list a file name and a pointer to the location on the hard drive.
Scandisk simply follows each directory entry to see if indeed the file that is supposed to be there is there. It would be like going through the index of a book, checking the subject, turning to the page, and seeing if the subject is covered on that page. The most common problem found by scandisk is pointers to the wrong place. It would be the same thing as turning to the page indicated in the index and the subject is not covered on that page. Usually, file corruption.