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For a job, I have to wipe a lot of disks that contain sensitive information. Those disks are all identical, I cannot wipe more than one at the same time, and fully wiping one takes me a full hour.

To gain some time, I though about stopping dd if=/dev/zero of=/dev/sda whenever it wrote enough data. All those disks are 1TB of capacity, but only 30 GB were used. So if I write ~30GB (or let's say 60GB to be sure), is it safe to consider that all data were safely erased ? I don't know much about where does ext4 allocate the files on the disk.

Thanksies !

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    What are you constraints, against what do you need to be safe? Wiping sensitive information in various setups typically is governed by some standards that describe how to do things or what is appropriate. I certainly think your problem is far more complex than just writing some GB at start of device (you have no guarantee that ext4 or anything else put things at start). See for example "A Comprehensive List of Data Wiping and Erasure Standards" at blancco.com/…. The "right" one surely depends on the industry/country you are in. – Patrick Mevzek Jun 18 '19 at 16:57
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If the disks were originally fully encrypted, then basically your problem is relatively easy....destruction of the key information will generally prevent the sensitive information from being accessible.

Are the disks SSDs? If so, then your process is faulty and will not destory all the sensitive information. The only way to destroy an UNencrypted SSD is physical destruction. Load leveling algorithms distribute data throughout the memory cells for wear balancing thus simply writing to the available space will not work.

Second, just wiping the active files and NOT the entire disk will result in sensitive information still remaining on the disk and recoverable by relatively easy means.

Also, simply writing zeros ONCE is not a good method of ensuring hard disks are clean. Several articles have been written indicating at LEAST three different patterns are needed to make it harder (not impossible) of sensitive data recovery.

Ultimately, what one company does over another is based on the sensitivity of the data involved and if the resulting device will be re-used internally or disposed of externally.

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