Suppose I have less than five per cent free disk space, so Defragger tells me about. So I remove some of my stuff, and the circle diagram to the right on the start page gets updated.
Nevertheless, the "Defrag" button still causes the dialog that complains about < 5% free space. After restart, though, it works without any warnings. That's not convenient to restart Defragger just to get rid of the pesky warning.
Hope I've helped to improve the product
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My Defragger version is 1.11.148, the platform is WinXP SP3 (not sure if it matters, though).