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I have always been told that I should back up my data before partitioning drives, but what are the reasons why the data would be lost?

Most of the time when partitioning drives, I take my largest partition and shrink it, and then I create a new partition with the space that I now have on my drive. What are the chances that I would lose data from my large partition when performing this operation?

Thanks!

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Data is not really lost, it is usually inaccessible until you restore the original partition structure if something does go wrong, it can become a complicated mess to correct, that is why you back up, really you should be backing up important data on a regular basis anyway. Forums are full of people who "lost" data by accident, some recover it some do not, lost forever. –  Moab Mar 19 '12 at 23:08
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There are different kinds of partitioning tools. Some partitioning tools (e.g. *nix fdisk, *BSD disklabel) only change where partitions start and end. If there are existing filesystems on the drive whose starts and ends overlap the new partition sizes, tough, you wreck the existing filesystems and lose the data. If you use these tools, then backups are essential if you want to have a copy of the data on disk.

Some newer and fancier partitioning tools, e.g. Mac OS's Disk Utility, are closely coupled with the local operating system. They have enough knowledge of the filesystems on disk that they can edit the filesystem, altering superblocks and summary information and whatever else, and moving data around to match the new partition sizes. Tools like this let you repartition a drive without losing data, mostly. You could still have a system crash or a power outage in the middle of the process and end up with a jumbled drive. Also, your partitioning utility could have a bug in it. You are gambling everything on the drive that the repartitioning process goes well. So even with the fancy tools, it is wise to have backed up your data.

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The recommendation to back up your data is because there was once a time when the tool you used to modify your partition tables could perhaps do something other than what you intended, either because of a bug with the software, a misunderstanding of the user, or simply by accident (a typo or some sort of wrong selection).

Depending on the result, you could end up with a disk that no longer contains your data.

Even today, most of these issues still remain. Any time you're editing the layout of your disk you should make the necessary precautions to secure your data.

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You might stumble on a bug of the partitioning tool, or the power might go out, or your computer can crash (very unlikely) during the process.

I have had Windows installations break after their partition was shrunk.

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