FAQ Using - Is it safe to stop MyDefrag?
Yes, it is safe to stop MyDefrag at any time. There is no risk of losing data or
corrupting your disk. Besides the "exit" button on the MyDefrag window you can also use
all the other ways to stop a Windows program, such as clicking the "x" in the top-right
corner of the window, pressing ALT-F4, via the pull-down menu in the taskbar,
or by killing the program via the task manager or another utility.
See also:
How safe is MyDefrag? I have stopped MyDefrag, but it remains running? Frequently Asked Questions