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I have two of the exact same 2TB external HDDs. I copied a bunch of information to one of them and then copied everything from the first to the second HDD and now their sitting at different amounts of free space. The copy job completed without error and there are no obvious large hidden files lying around.

First question is: how can this happen? I used robocopy so I know I brought over all hidden files and everything

Second question is: is there anything I can use to make sure both HDDs have exactly the same information on them?

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I was about to suggest RAID 1, but I don't think you can do that as external hard drives... – Canadian Luke Jan 27 '12 at 0:39
If you go to My Computer -> right click drive and select 'Properties', do both drives list exactly the same amount of bytes in size? In either case there are a number of reasons why even identical drives can show slight deviations in available space. You also didn't mention how large of a space difference you're referring to, what I'm talking to usually equates to anywhere from a few bytes to a couple hundred KB. – Garrett Jan 27 '12 at 1:00

Several possible causes

  • System restore or volume shadow copy using the extra space
  • chkdsk found some file fragments and left them in some folder
  • MFT size difference
  • Cluster size difference (more obvious if you're had a lot of small files)
  • Robocopy did not handle sparse files/compressed files/junctions correctly
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Robocopy does not copy hardlinks over. Rather the target file is copied. – surfasb Jan 27 '12 at 5:14

You could use SyncToy from Microsoft, or ROBOCOPY with the /MIR command under Windows XP or higher. What OS are you using?

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