need help with disk defragment

it was my first time to run the disk defragmenter in my laptop (i have bought it for 3 years), but something seems wrong with the disk defragmenter, it kept showing the message "drive contents changed, restarting", i left it runing overnight, but next morning it's still runing and giving the same message. what's wrong with this? any one got experence? thank you very much!!!

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