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I had a serious failure on a 2TB external HDD this week. Yosemite says the drive is "not readable by this computer."

Oh well, it happens to all HDDs sooner or later.

Even though the contents could be re-downloaded from the internet, I decided to run Disk Drill to see how much I could recover.

What seems strange to me is that Disk Drill says it has "reconstructed" 2.4 TB of files. enter image description here

While the HDD only has 2 TB capacity.

enter image description here

I'm simply curious, how is this technically possible? How can DD have reconstructed more data than could possibly have been stored on the drive? I don't require a solution to any technical problem. I'm just curious about the technical details behind this.

FWIW, prior to the failure, there was a 1 TB encrypted HFS+ Time Machine backup partition, a 600 GB HFS+ unencrypted partition, and a 400 GB unencrypted NTFS partition.

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  • Contents of compressed files that are no longer compressed is one possibility. PS - Noticed you are attempting rescue on Seagate. Been there many times. Switched to WD or HGST and my drives issues went down significantly. – Allan Jul 20 '16 at 19:08
  • It would also attempt to collect data from ALL available nodes - it doesn't mean you got 2 or 2.4 GB of actually valid data back - just that's what it thinks it might have scavenged. Recovering from a backup is far more reliable, really :/ – Tetsujin Jul 20 '16 at 19:15
  • @Tetsujin Ah, I see. Yeah, I guess the most reliable thing to do would be to just download everything again from the online source from which it originally came. – Fiksdal Jul 20 '16 at 19:19
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    Another possible reason is sparse files. Sparse files are files with "holes" in them - i.e. longer stretches of zeroes. On disk they can be represented in a much more compact manner on some file systems, by indicating just the number of zero-bytes to follow instead of having the actual bytes. When converted to ordinary files, they will "blow up" to their actual size, if the recovery tools does not support saving them as sparse files again. – jksoegaard Jul 21 '16 at 8:30
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It looks like Disk Drill has found the same files multiple times. For example, here the program says it has found four individual .iso files of 6.36 GB. In reality, the original medium only had one such file.

enter image description here

This would explain why the total size is several times as large as the original amount of data.

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