you have to enter safe mode first, then scan the drive to
correct errors, then defrag.  How to do this will vary
depending on your operating system.  More can be found at
Dell KNowledge Base

Get the free ScanDefrag. It is the most advanced disk maintenance
program there is. It runs Disk Cleanup, ScanDisk, and Disk
Defragmenter. It's easy to setup and use, and it can deal with
problems like "drive's contents have changed: restarting...".
Get it and read about all the options it has that you can use.
http://home.earthlink.net/~bblanton2/scandefrag/main.htm
http://www.blueorbsoft.com/scandefrag/index.html

It is not unusual to see Defrag restart a number of times.
It is , however, unusual to see ScanDisk restart.
This is why I suggest starting it at bedtime and letting it run overnight.
Please, email "The ScanDefrag Team" if you have any problems using our program. Your problem, will be properly attended to.  And we assure you, that every attempt will be made to resolved it..
To see how to email us, read the ScanDefrag "Readme" file, using ScanDefrag under the "Help" menu.