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Booted up from live media (ubuntu 12.04) with HDD (3TB, GUID, single EXT4 partition used for storing data and nothing else)connected via SATA. For some reason the file system was only allowing "read only" access so I decided to unmount. I opened up "disk utility" and just clicked "mount" for the heck of it. For whatever reason it hung (and didn't actually end up finishing mount)... So I got impatient and did a sysreq+REISUO thing, but the system never shut off.. Got impatient again and just pulled the plug. Rebooted to find my that there is no longer any partition found on the device.

So basically.. I damaged, corrupted, destroyed, whatever my ext4 filesystem..? obviously all the data is still sitting there..right? I'm prob gonna go ahead and write block + dd the whole thing before before I do anything else, just 'cause.. when I get a hold of a spare 3 TB drive in the near future... that said.. what do I do, if anything, to get my partition and all the data on that partition back? What happened? How do I go about not losing 3 TB worth of data that I hadn't backed up yet?

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It's possible that TestDisk will recover the partition on the disk. If not, you may need to resort to PhotoRec, which is a file-level recovery tool. The trouble with PhotoRec is that it doesn't typically recover filenames or directory structures, so you can end up spending dozens of hours sorting through and renaming/moving files that it recovers.

As to the cause, it's impossible to know. Some possibilities that spring to mind include:

  • Random data corruption caused by bugs, cosmic rays (for real), a power surge, a bad cable, or whatever.
  • A physically damaged disk. Checking it with a SMART utility (like GSMartControl) is advisable.
  • The uncontrolled shutdown you describe may have caused data corruption because a cache wasn't written back to disk.
  • Somebody may have broken into your computer and maliciously done damage. (Similarly, a virus or worm might be responsible, especially if the computer dual-boots with Windows.)

No doubt I'm forgetting some possibilities, too. You can perform checks like a SMART test, tests on your cables, security screenings, etc., to look for some of these items. Others, if they're the cause, will likely remain mysteries forever.

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