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A strategy for the effective erasure of data is a critical component of managing data securely. Deleting files using operating system tools or reformatting a hard drive will not irretrievably destroy the data. A number of software programs make recovering such deleted data simple. Specialised file ‘shredding’ or ‘secure erase’ tools can reduce the risk of recovery to an acceptable level and may be used for moderately sensitive data.
Please note: the only way to guarantee that data is unrecoverable is to physically destroy the media it is stored on.
Hard Drives
Traditional hard drives are magnetic storage devices and simply deleting a file does not remove it from the drive. Files need to be overwritten repeatedly to ensure they are practically unrecoverable. There are various software programs available that can securely delete files from hard drives. If a hard disk is already encrypted, it is not necessary to delete data on it as long as the decryption key remains private and protected. Deletion by this method is quicker than deletion using other tools. For Hard-Disks that are not encrypted, the use of tools like Darik’s Boot And Nuke (DBAN) is recommended.
Solid-state Disks (SSDs), USB Flash Drives, and SD Cards
Solid-state hard disks (SSD) and USB flash drives (memory sticks) use a technology called as ‘wear levelling’. Therefore, the methods for securely erasing files mentioned above cannot be relied upon. To erase SSD drives, use the manufacturer’s delete utilities if available.
Securely erasing a USB flash drive is a complex procedure which involves formatting the drive and encrypting it with a sufficiently large passphrase. The UK Data Service recommends using a 50-character passphrase. However, this may require the use of additional tools that may not be available. It is recommended to use Full-Disk Encryption on such media if it is available.
Paper
Shredders certified to an appropriate security level should be used for destroying paper and CD/DVD discs.
Optical Disks
Never throw away an optical disk unless you are absolutely sure that there is no sensitive data on it. The use of a shredder is recommended.
Floppy Disks and Tapes
Magnetic storage devices such as floppy disks and tape can be securely erased by a process of demagnetisation such as using a degausser.
Further reading: https://www.ncsc.gov.uk/guidance/secure-sanitisation-storage-media {external link}
Version 2.0
Last updated: 14-Jan-2021