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I have a 1TB hdd, which I would like to encrypt. I would like to make a hidden volume, with almost nothing within but some decoy data, and the rest in a hidden volume.

However, my driver is over 95% full. Is it still possible to do this, or would it have to be done on an empty drive, and then copy the data over?

I could not find the answer to this question in the documentation.

Also, how easy would it be to undo, or unencrypt the drive? Would it again need another empty drive to begin with?

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Encrypting more than 950MB will take a hell of a time. And since this is the system drive, what will you do if it fails and bricks your computer? –  harrymc Oct 14 '09 at 19:42
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Surely it wouldn't brick the computer - you can always format and reinstall. –  Blorgbeard Oct 14 '09 at 20:19
    
It is not the system drive, it is an external harddrive. Sorry for not specifying this. Just how long would it take? –  Bill Grey Oct 14 '09 at 20:32

2 Answers 2

From the Truecrypt FAQ:

Can I encrypt a partition/drive without losing the data currently stored on it?

Yes, but the following conditions must be met:

  • If you want to encrypt an entire system drive (which may contain multiple partitions) or a system partition (in other words, if you want to encrypt a drive or partition where Windows is installed), you can do so provided that you use TrueCrypt 5.0 or later and that you use Windows XP or a later version of Windows (such as Windows Vista) (select 'System' > 'Encrypt System Partition/Drive' and then follow the instructions in the wizard).

  • If you want to encrypt a non-system partition in place, you can do so provided that it contains an NTFS filesystem, that you use TrueCrypt 6.1 or later, and that you use Windows Vista or a later version of Windows (for example, Windows 7) (click 'Create Volume' > 'Encrypt a non-system partition' > 'Standard volume' > 'Select Device' > 'Encrypt partition in place' and then follow the instructions in the wizard).

Steps to remove the encryption are described here:

Please note that TrueCrypt can in-place decrypt only system partitions and system drives (select System > Permanently Decrypt System Partition/Drive). If you need to remove encryption (e.g., if you no longer need encryption) from a non-system volume, please follow these steps:

...

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Thanks, I did actually miss the FAQ. But, it does not say about making a hidden volume, or undoing the process –  Bill Grey Oct 14 '09 at 19:44
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Also, would non-system partition also refer to non-system drive? Is there any indication of how long this may take? –  Bill Grey Oct 14 '09 at 19:45

Sadly, no, you can't....

Tested with TrueCrypt 7.1a: If you go to 'Create Volume' > 'Encrypt a non-system partition' > 'Hidden volume' you have two options:

Normal mode: You have a nice:

WARNING: Note that the partition/device will be formatted and all data currently stored on it will be lost.

Direct mode:

If you select this option, you will create a hidden volume within an existing TrueCrypt volume.

The latter case is not a choice, since the partition is not yet encrypted.

So, you can't! At best you could encrypt the partition in normal mode, then create a hidden volume within it, and then transfer the data from the normal volume to the hidden volume... but since you can't resize a hidden volume, you will loose the size of your data in drive space. Certainly a bad choice. Moreover the normal volume would have to be wiped after this operation to preserve plausible deniability.

So the best choice is sadly to copy everything to a temporary location (provided that you have the space to do so), create the hidden volume, and finally to copy the data back to the partition. Don't forget to wipe to temporary location or to use a temporary truecrypt volume in this temporary location.

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