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But not an effective one. Generally defrag routines copy data and then update the file system. Power loss would have to occur during the file system write in order for it to be destructive. (And in the case of journaling file systems, likely still recoverable.)

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Yes, but the system is writing roughly half the time, and defragging was mostly a thing before the advent of journalling file systems.

Power loss while a defragmenter is writing file data will not cause data loss, even on the most basic of file systems like FAT or HFS. Duplicating data into free space is effectively a no-op until the file system is updated to point from the old location to the new location.

It think most versions of windows that included a defragmenter as part of the OS were XP and later, all of which would have used NTFS, which is a journaling filesystem.

DOS defrag goes back a lot farther than that. It even predates FAT32, let alone NTFS.

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