Hmm. Perhaps a benefit of a backup on device level... Defrag must not change a content of your file - but it definitely might put it into...
I'd rather call this a weakness. From other hand a backup software that can work both on filesystem and 'raw' block device level most likely...
They might do an increment related to an initial full backup, not to a 'last state'. As I understand backup files are in some proprietary format...
Because you paid for a <snip> of bugs? 'Sad but true'....
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