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From a (very long, but definitely worth to read) article on SSDs:

When you delete a file in your OS, there is no reaction from either a hard drive or SSD. It isn’t until you overwrite the sector (on a hard drive) or page (on a SSD) that you actually lose the data. File recovery programs use this property to their advantage and that’s how they help you recover deleted files.

The key distinction between HDDs and SSDs however is what happens when you overwrite a file. While a HDD can simply write the new data to the same sector, a SSD will allocate a new (or previously used) page for the overwritten data. The page that contains the now invalid data will simply be marked as invalid and at some point it’ll get erased.

So, what would be the best way to securely erase files stored on a SSD? Overwriting with random data as we are used to from hard disks (e.g. using the "shred" utility) won't work unless you overwrite the WHOLE drive...

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5 Answers 5

How about encrypting the volume itself? The deleted data will still be there but should be inaccessible to anyone without the appropriate key.

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Funny I didn't think about that - as I always use LUKS encryption on my hard disks anyway ;) –  c089 Aug 13 '09 at 11:19
    
Good idea teabot. Thanks. –  JamesC Dec 31 '09 at 10:02
    
As stated here this will have serious performance impacts. With SandForce-based drives, using ATA secure erase will wipe the AES encryption key, providing a solution to the secure delete problem when not using software encryption. –  c089 Feb 27 '11 at 12:10

Even if you overwrite the whole drive, you cannot be absolutely certain since both HDs and SSDs contain spare sectors that are substituted when failures occur, or in the case of SSDs to aid wear levelling.

If you want to be absolutely certain the data is irrecoverable gone, you have to physically destroy the drive beyond recovery. Overwriting gives you reasonable certainty on HDs - on SSDs, there isn't really a way to achieve even that. If you fill the whole drive, it's still possible that the block the data was in has been switched to a spare block for wear levelling and will reappear later.

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If the SSD drive supports the ATA security mode feature set, then it has secure erase functionality built in, which you should be able to access using something like Secure Erase.

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+1 for ATA security mode feature set. There's a good video on this on SecurityTube.net –  RJFalconer Aug 13 '09 at 14:30
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But that feature is only for complete drives as well, isnt it? –  c089 Aug 14 '09 at 14:23
    
@Chris089: That's correct: secure erase will erase the whole drive. Overwriting the entire drive with random data is not guaranteed to erase the whole drive, due to underprovisioning. (But it could make some of the unerased data inaccessible to ordinary users, at least.) See anandtech.com/storage/showdoc.aspx?i=3531&p=9 for more info. –  bk1e Aug 14 '09 at 16:46

As you thought, wiping all the free space to ensure that any blocks that used to contain the data are wiped is the only way to be sure. Well, other than taking of an nuking the site from orbit...

This will be quite time consuming, and will reduce the life of the flash chips, if you use a multi-pass shred.

The other way to protected the deleted data is to store it encrypted in the first places using an encrypted filesystem using something like truecrypt.

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up vote 3 down vote accepted

As stated by @teabot using full disk encryption software will circumvent the issue of secure deletion because you don't have to any more. However, as stated in a related question, this will have huge performance impacts because this prevents a lot of the controller's features like compression and TRIM and has been reported to decrease the performance up to a level where a normal hard disk is faster then the expensive SSD. For SandForce-based devices, there is a better solution: Those devices do AES encryption by default and the key will be wiped when you use the ATA secure delete feature, making all data unaccessible unless the attacker can break AES (128bit for current, 256bit for the new drives released in march 2011).

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However, there doesn't seem to be a solutio for securely deleting just a few files. You have to erase the whole disk. –  c089 Mar 1 '11 at 7:26

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