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In Windows how do I determine how many bytes have changed on a given Hard Disk??? I don't just mean how many files changed and the sizes of those changes. I mean given a period of time, say 1 hour, what is the size of the changes written to disk?

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There's no built-in mechanism for the Windows OSs you mention in your tags to do what you're asking and be byte-accurate. A driver for a given disk controller might support that (and I know some RAID controllers do), but that's as close as you're going to get to byte-accurate.

You could monitor the "write" counters on the PhysicalDisk performance object for the disk in question and probably get an approximate count, but it's not going to be byte-accurate. Fire up PERFMON and have a look.

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There's probably a way thru one of the raw perfmon counters availabe via WMI see WMI performance counter types. It'd be helpful to know why you want this data, as there may be other ways to get it (per process etc)

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If you want accurate result (but not necessarily fast), you can do partition level image backups, and then write a tool that will compare them cluster by cluster or whatever resolution for change detection you want.

Important feature that you want from the OS and the file system is volume snapshotting, so that you take atomic snapshot of image no matter what's being written to disk while you're copying that image. Windows+NTFS supports that. ZFS, UFS, ext4 support that. FAT32 and ext3 do not support that.

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